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Copyright, 1945, by H. D. Smyth 

Reproduction in whole or in part 
authorized and permitted 

Printed in the United States of America 
by Maple Press, York, Pennsylvania 


THE story of the development of the atomic bomb by the 
combined efforts of many groups in the United States is a 
fascinating but highly technical account of an enormous enter- 
prise. Obviously military security prevents this story from being 
told in full at this time. However, there is no reason why the 
administrative history of the Atomic Bomb Project and the basic 
scientific knowledge on which the several developments were 
based should not be available now to the general public. To this 
end this account by Professor H. D. Smyth is presented. 

All pertinent scientific information which can be released to 
the public at this time without violating the needs of national 
security is contained in this volume. No requests for additional 
information should be made to private persons or organizations 
associated directly or indirectly with the project. Persons dis- 
closing or securing additional information by any means whatso- 
ever without authorization are subject to severe penalties under 
the Espionage Act. 

The success of the development is due to the many thousands 
of scientists, engineers, workmen and administrators both 
civilian and military whose prolonged labor, silent perseverance, 
and whole-hearted cooperation have made possible the un- 
precedented technical accomplishments here described. 


Major General, USA 
War Department 
Washington, D. C. 
August 1945 


THE ultimate responsibility for our nation's policy rests on 
its citizens and they can discharge such responsibilities wisely 
only if they are informed. The average citizen cannot be expected 
to understand clearly how an atomic bomb is constructed or how 
it works but there is in this country a substantial group of engi- 
neers and scientific men who can understand such things and 
who can explain the potentialities of atomic bombs to their fellow 
citizens. The present report is written for this professional group 
and is a matter-of-fact, general account of work in the United 
States since 1939 aimed at the production of such bombs. It is 
neither a documented official history nor a technical treatise for 
experts. Secrecy requirements have affected both the detailed 
content and general emphasis so that many interesting develop- 
ments have been omitted. 

References to British and Canadian work are not intended to 
be complete since this is written from the point of view of the 
activities in this country. 

The writer hopes that this account is substantially accurate, 
thanks to cooperation from all groups in the project; he takes 
full responsibility for such errors as may occur. 


July /, 1945 

Minor changes have been made for this edition. These changes 
consist of the following variations from the report as issued 
August 12, 1945: (1) Minor clarifications and corrections in 
wording; (2) Inclusion of a paragraph on radioactive effects 
issued by the War Department to accompany the original release 

viii Preface 

of this report; (3) Addition of a few sentences on the success of 
the health precautions; (4) Addition of a few names; (5) Addition 
of Appendix 6, giving the War Department release on the New 
Mexico test of July 16, 1945; (6) Inclusion of the photographic 
section; (7) Inclusion of an index. 

H. D. S. 

September 7, 1945 







DECEMBER 1941 45 



CHICAGO IN 1942 88 


AS OF FEBRUARY 1943 108 


TO JUNE 1945 130 














1.1. The purpose of this report is to describe the scientific and 
technical developments in this country since 1940 directed toward 
the military use of energy from atomic nuclei. Although not 
written as a "popular" account of the subject, this report is 
intended to be intelligible to scientists and engineers generally 
and to other college graduates with a good grounding in physics 
and chemistry. The equivalence of mass and energy is chosen as 
the guiding principle in the presentation of the background 
material of the "Introduction." 


1.2. There are two principles that have been cornerstones of 
the structure of modern science. The first that matter can be 
neither created nor destroyed but only altered in form was 
enunciated in the eighteenth century and is familiar to every 
student of chemistry; it has led to the principle known as the law 
of conservation of mass. The second that energy can be neither 
created nor destroyed but only altered in form emerged in the 
nineteenth century and has ever since been the plague of in- 
ventors of perpetual-motion machines; it is known as the law of 
conservation of energy. 

1.3. These two principles have constantly guided and disci- 
plined the development and application of science. For all 
practical purposes they were unaltered and separate until some 
five years ago. For most practical purposes they still are so, but 
it is now known that they are, in fact, two phases of a single 
principle for we have discovered that energy may sometimes be 
converted into matter and matter into energy. Specifically, such 
a conversion is observed in the phenomenon of nuclear fission of 


2 Introduction 

uranium, a process in which atomic nuclei split into fragments 
with the release of an enormous amount of energy. The military 
use of this energy has been the object of the research and pro- 
duction projects described in this report. 


1.4. One conclusion that appeared rather early in the develop- 
ment of the theory of relativity was that the inertial mass of a 
moving body increased as its speed increased. This implied an 
equivalence between an increase in energy of motion of a body, 
that is, its kinetic energy, and an increase in its mass. To most 
practical physicists and engineers this appeared a mathematical 
fiction of no practical importance. Even Einstein could hardly 
have foreseen the present applications, but as early as 1905 he 
did clearly state that mass and energy were equivalent and sug- 
gested that proof of this equivalence might be found by the study 
of radioactive substances. He concluded that the amount of 
energy, E, equivalent to a mass, m, was given by the equation 

E = mc 2 

where c is the velocity of light. If this is stated in actual numbers, 
its startling character is apparent. It shows that one kilogram 
(2.2 pounds) of matter, if converted entirely into energy, would 
give 25 billion kilowatt hours of energy. This is equal to the 
energy that would be generated by the total electric power 
industry in the United States (as of 1939) running for approxi- 
mately two months. Compare this fantastic figure with the 8.5 
kilowatt hours of heat energy which may be produced by burning 
an equal amount of coal. 

1.5. The extreme size of this conversion figure was interesting 
in several respects. In the first place, it explained why the equiva- 
lence of mass and energy was never observed in ordinary chemical 
combustion. We now believe that the heat given off in such a 
combustion has mass associated with it, but this mass is so small 
that it cannot be detected by the most sensitive balances avail- 

Introduction 3 

able. (It is of the order of a few billionths of a gram per mole.) 
In the second place, it was made clear that no appreciable 
quantities of matter were being converted into energy in any 
familiar terrestrial processes, since no such large sources of energy 
were known. Further, the possibility of initiating or controlling 
such a conversion in any practical way seemed very remote. 
Finally, the very size of the conversion factor opened a magnifi- 
cent field of speculation to philosophers, physicists, engineers, 
and comic-strip artists. For twenty-five years such speculation 
was unsupported by direct experimental evidence, but beginning 
about 1 930 such evidence began to appear in rapidly increasing 
quantity. Before discussing such evidence and the practical partial 
conversion of matter into energy that is our main theme, we shall 
review the foundations of atomic and nuclear physics. General 
familiarity with the atomic nature of matter and with the exist- 
ence of electrons is assumed. Our treatment will be little more 
than an outline which may be elaborated by reference to books 
such as Pollard and Davidson's Applied Nuclear Physics and Strana- 
than's The "Particles" of Modern Physics. 


1.6. First discovered by H. Becquerel in 1896 and subsequently 
studied by Pierre and Marie Curie, E. Rutherford, and many 
others, the phe'nomena of radioactivity have played leading roles 
in the discovery of the general laws of atomic structure and in the 
verification of the equivalence of mass and energy. 


1.7. The first phenomenon of radioactivity observed was the 
blackening of photographic plates by uranium minerals. Al- 
though this effect is still used to some extent in research on radio- 
activity, the property of radioactive substances that is of greatest 
scientific value is their ability to ionize gases. Under normal con- 
ditions air and other gases do not conduct electricity otherwise 
power lines and electrical machines would not operate in the 

4 Introduction 

open as they do. But under some circumstances the molecules 
of air are broken apart into positively and negatively charged 
fragments, called ions. Air thus ionized does conduct electricity. 
Within a few months after the first discovery of radioactivity 
Becquerel found that uranium had the power to ionize air. 
Specifically he found that the charge on an electroscope would 
leak away rapidly through the air if some uranium salts were 
placed near it. (The same thing would happen to a storage 
battery if sufficient radioactive material were placed near by.) 
Ever since that time the rate of discharge of an electroscope has 
served as a measure of intensity of radioactivity. Furthermore, 
nearly all present-day instruments for studying radioactive phe- 
nomena depend on this ionization effect directly or indirectly. 
An elementary account of such instruments, notably electro- 
scopes, Geiger-Muller counters, ionization chambers, and Wilson 
cloud chambers is given in Appendix 1. 


1.8. Evidence that different radioactive substances differ in 
their ionizing power both in kind and in intensity indicates that 
there are differences in the "radiations" emitted. Some of the 
radiations are much more penetrating than others; consequently, 
two radioactive samples having the same effect on an "un- 
shielded" electroscope may have very different effects if the 
electroscope is "shielded," i.e., if screens are interposed between 
the sample and the electroscope. These screens are said to absorb 
the radiation. 

1.9. Studies of absorption and other phenomena have shown 
that in fact there are three types of "radiation" given off by 
radioactive substances. There are alpha particles, which are 
high-speed ionized helium atoms (actually the nuclei of helium 
atoms), beta particles, which are high-speed electrons, and 
gamma rays, which are electromagnetic radiations similar to 
X-rays. Of these only the gamma rays are properly called radia- 
tions, and even these act very much like particles because of 
their short wave-length. Such a "particle" or quantum of gamma 

Introduction 5 

radiation is called a photon. In general, the gamma rays are very 
penetrating, the alpha and beta rays less so. Even though the 
alpha and beta rays are not very penetrating, they have enormous 
kinetic energies for particles of atomic size, energies thousands 
of times greater than the kinetic energies which the molecules of 
a gas have by reason of their thermal motion, and thousands 
of times greater than the energy changes per atom in chemical 
reactions. It was for this reason that Einstein suggested that 
studies of radioactivity might show the equivalence of mass and 


1.10. Before considering what types of atoms emit alpha, beta 
and gamma rays, and before discussing the laws that govern such 
emission, we shall describe the current ideas on how atoms are 
constructed, ideas based partly on the study of radioactivity. 

1.11. According to our present view every atom consists of a 
small heavy nucleus approximately 10~~ 12 cm in diameter sur- 
rounded by a largely empty region 10~ 8 cm in diameter in which 
electrons move somewhat like planets about the sun. The nucleus 
carries an integral number of positive charges, each 1.6 X 10~ 19 
coulombs in size. (See Appendix 2 for a discussion of units.) 
Each electron carries one negative charge of this same size, and 
the number of electrons circulating around the nucleus is equal 
to the number of positive charges on the nucleus so that the atom 
as a whole has a net charge of zero. 

1.12. Atomic Number and Electronic Structure. The number of 
positive charges in the nucleus is called the atomic number, Z. 
It determines the number of electrons in the extranuclear struc- 
ture, and this in turn determines the chemical properties of the 
atom. Thus all the atoms of a given chemical element have the 
same atomic number, and conversely all atoms having the same 
atomic number are atoms of the same element regardless of 
possible differences in their nuclear structure. The extranuclear 
electrons in an atom arrange themselves in successive shells 
according to well-established laws. Optical spectra arise from 

6 Introduction 

disturbances in the outer parts of this electron structure; X-rays 
arise from disturbances of the electrons close to the nucleus. The 
chemical properties of an atom depend on the outermost elec- 
trons, and the formation of chemical compounds is accompanied 
by minor rearrangements of these electronic structures. Conse- 
quently, when energy is obtained by oxidation, combustion, 
explosion, or other chemical processes, it is obtained at the 
expense of these structures so that the arrangement of the elec- 
trons in the products of the process must be one of lowered energy 
content. (Presumably the total mass of these products is corre- 
spondingly lower but not detectably so.) The atomic nuclei are 
not affected by any chemical process. 

1.13. Mass Number. Not only is the positive charge on a nucleus 
always an integral number of electronic charges, but the mass 
of the nucleus is always approximately a whole number times a 
fundamental unit of mass which is almost the mass of a proton, 
the nucleus of a hydrogen atom. (See Appendix 2.) This whole 
number is called the mass number, A, and is always at least twice 
as great as the atomic number except in the cases of hydrogen 
and a rare isotope of helium. Since the mass of a proton is about 
1,800 times that of an electron, the mass of the nucleus is very 
nearly the whole mass of the atom. 

1.14. Isotopes and Isobars. Two species of atoms having the 
same atomic number but different mass numbers are called 
isotopes. They are chemically identical, being merely two species 
of the same chemical element. If two species of atoms have the 
same mass number but different atomic numbers, they are called 
isobars and represent two different chemical elements. 


1.15. If an atom emits an alpha particle (which has an atomic 
number of two and a mass of four), it becomes an atom of a 
different element with an atomic number lower by two and a 
mass number lower by four. The emission by a nucleus of a beta 
particle increases the atomic number by one and leaves the mass 
number unaltered. In some cases, these changes are accompanied 

Introduction 7 

by the emission of gamma rays. Elements which spontaneously 
change or "disintegrate" in these ways are unstable and are 
described as being "radioactive." The only natural elements 
which exhibit this property of emitting alpha or beta particles 
are (with a few minor exceptions) those of very high atomic 
numbers and mass numbers, such as uranium, thorium, radium, 
and actinium, i.e., those known to have the most complicated 
nuclear structures. 


1.16. All the atoms of a particular radioactive species have the 
same probability of disintegrating in a given time, so that an 
appreciable sample of radioactive material, containing many 
millions of atoms, always changes or "disintegrates" at the same 
rate. This rate at which the material changes is expressed in 
terms of the "half-life," the time required for one half the atoms 
initially present to disintegrate, which evidently is constant for 
any particular atomic species. Half-lives of radioactive materials 
range from fractions of a second for the most unstable to billions 
of years for those which are only slightly unstable. Often, the 
"daughter" nucleus like its radioactive "parent" is itself radio- 
active and so on down the line for several successive generations 
of nuclei until a stable one is finally reached. There are three 
such families or series comprising all together about forty different 
radioactive species. The radium series starts from one isotope of 
uranium, the actinium series from another isotope of uranium, 
and the thorium series from thorium. The final product of each 
series, after ten or twelve successive emissions of alpha and beta 
particles, is a stable isotope of lead. 


1.17. Before 1919 no one had succeeded in disturbing the 
stability of ordinary nuclei or affecting the disintegration rates 
of those that were naturally radioactive. In 1919 Rutherford 
showed that high-energy alpha particles could cause an alteration 


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Introduction 9 

in the nucleus of an ordinary element. Specifically he succeeded 
in changing a few atoms of nitrogen into atoms of oxygen by 
bombarding them with alpha particles. The process involved 
may be written as 

He 4 + N 14 -i O 17 + H 1 

meaning that a helium nucleus of mass number 4 (an alpha 
particle) striking a nitrogen nucleus of mass number 14 produces 
an oxygen nucleus of mass number 17 and a hydrogen nucleus 
of mass number 1. The hydrogen nucleus, known as the "proton," 
is of special importance since it has the smallest mass of any 
nucleus. Although protons do not appear in natural radioactive 
processes, there is much direct evidence that they can be knocked 
out of nuclei. 


1.18. In the decade following Rutherford's work many similar 
experiments were performed with similar results. One series of 
experiments of this type led to the discovery of the neutron, 
which will be discussed in some detail since the neutron is prac- 
tically the theme song of this whole project. 

1.19. In 1930 W. Bothe and H. Becker in Germany found that 
if the very energetic natural alpha particles from polonium fell 
on certain of the light elements, specifically beryllium, boron or 
lithium, an unusually penetrating radiation was produced. At 
first this radiation was thought to be gamma radiation although 
it was more penetrating than any gamma rays known, and the 
details of experimental results were very difficult to interpret on 
this basis. The next important contribution was reported in 1932 
by Irene Curie and F. Joliot in Paris. They showed that if this 
unknown radiation fell on paraffin or any other hydrogen- 
containing compound it ejected protons of very high energy. 
This was not in itself inconsistent with the assumed gamma-ray 
nature of the new radiation, but detailed quantitative analysis of 
the data became increasingly difficult to reconcile with such an 
hypothesis. Finally (later in 1932) J. Chadwick in England per- 

10 Introduction 

formed a series of experiments showing that the gamma-ray 
hypothesis was untenable. He suggested that in fact the new 
radiation consisted of uncharged particles of approximately the 
mass of the proton, and he performed a series of experiments 
verifying his suggestion. Such uncharged particles are now called 

1.20. The one characteristic of neutrons which differentiates 
them from other subatomic particles is the fact that they are 
uncharged. This property of neutrons delayed their discovery, 
makes them very penetrating, makes it impossible to observe 
them directly, and makes them very important as agents in 
nuclear change. To be sure, an atom in its normal state is also 
uncharged, but it is ten thousand times larger than a neutron and 
consists of a complex system of negatively charged electrons 
widely spaced around a positively charged nucleus. Charged 
particles (such as protons, electrons, or alpha particles) and 
electromagnetic radiations (such as gamma rays) lose energy in 
passing through matter. They exert electric forces which ionize 
atoms of the material through which they pass. (It is such ioniza- 
tion processes that make the air electrically conducting in the 
path of electric sparks and lightning flashes.) The energy taken 
up in ionization equals the energy lost by the charged particle, 
which slows down, or by the gamma ray, which is absorbed. The 
neutron, however, is unaffected by such forces; it is affected only 
by a very short-range force, i.e., a force that comes into play 
when the neutron comes very close indeed to an atomic nucleus. 
This is the kind of force that holds a nucleus together in spite of 
the mutual repulsion of the positive charges in it. Consequently 
a free neutron goes on its way unchecked until it makes a 
"head-on" collision with an atomic nucleus. Since nuclei are 
very small, such collisions occur but rarely and the neutron 
travels a long way before colliding. In the case of a collision of 
the "elastic" type, the ordinary laws of momentum apply as 
they do in the elastic collision of billiard balls. If the nucleus that 
is struck is heavy, it acquires relatively little speed, but if it is a 
proton, which is approximately equal in mass to the neutron, 

Introduction 1 1 

it is projected forward with a large fraction of the original speed 
of the neutron, which is itself correspondingly slowed. Secondary 
projectiles resulting from these collisions may be detected, for 
they are charged and produce ionization. The uncharged nature 
of the neutron makes it not only difficult to detect but difficult to 
control. Charged particles can be accelerated, decelerated, or 
deflected by electric or magnetic fields which have no effect on 
neutrons. Furthermore, free neutrons can be obtained only from 
nuclear disintegrations; there is no natural supply. The only 
means we have of controlling free neutrons is to put nuclei in 
their way so that they will be slowed and deflected or absorbed 
by collisions. As we shall see, these effects are of the greatest 
practical importance. 


1.21. The year 1932 brought the discovery not only of the 
neutron but also of the positron. The positron was first observed 
by C. D. Anderson at the California Institute of Technology. It 
has the same mass and the same magnitude of charge as the 
electron, but the charge is positive instead of negative. Except 
as a particle emitted by artificially radioactive nuclei, it is of 
little interest to us. 

1.22. One other major discovery marked the year 1932. H. C. 
Urey, F. G. Brickwedde, and G. M. Murphy found that hydrogen 
had an isotope of mass number 2, present in natural hydrogen 
to one part in 5,000. Because of its special importance this heavy 
species of hydrogen is given a name of its own, deuterium, and 
the corresponding nucleus is called the deuteron. Like the alpha 
particle the deuteron is not one of the fundamental particles but 
does play an important role in certain processes for producing 
nuclear disintegration. 


1.23. The idea that all elements are made out of a few funda- 
mental particles is an old one. It is now firmly established. We 
believe that there are three fundamental particles the neutron, 

12 Introduction 

the proton, and the electron. A complete treatise would also 
discuss the positron, which we have mentioned, the neutrino and 
the mesotron. The deuteron and alpha particle, which have 
already been mentioned, are important complex particles. 

1.24. According to our present views the nuclei of all atomic 
species are made up of neutrons and protons. The number of 
protons is equal to the atomic number, Z. The number of neu- 
trons, N, is equal to the difference between the mass number and 
the atomic number, or A Z. There are two sets of forces 
acting on these particles, ordinary electric coulomb forces of 
repulsion between the positive charges and very short-range 
forces between all the particles. These last forces are only 
partly understood, and we shall not attempt to discuss them. 
Suffice it to say that combined effects of these attractive and 
repulsive forces are such that only certain combinations of 
neutrons and protons are stable. If the neutrons and protons are 
few in number, stability occurs when their numbers are about 
equal. For larger nuclei, the proportion of neutrons required for 
stability is greater. Finally, at the end of the periodic table, 
where the number of protons is over 90 and the number of 
neutrons nearly 1 50, there are no completely stable nuclei. (Some 
of the heavy nuclei are almost stable as evidenced by very long 
half-lives.) If an unstable nucleus is formed artificially by adding 
an extra neutron or proton, eventually a change to a stable form 
occurs. Strangely enough, this is not accomplished by ejecting a 
proton or a neutron but by ejecting a positron or an electron; 
apparently within the nucleus a proton converts itself into a 
neutron and positron (or a neutron converts itself into a proton 
and electron), and the light charged particle is ejected. In other 
words, the mass number remains the same but the atomic number 
changes. The stability conditions are not very critical so that for 
a given mass number, i.e., given total number of protons and 
neutrons, there may be several stable arrangements of protons 
and neutrons (at most three or five) giving several isobars. For a 
given atomic number, i.e., given number of protons, conditions 
can vary still more widely so that some of the heavy elements 

Introduction 13 

have as many as ten or twelve stable isotopes. Some two hundred 
and fifty different stable nuclei have been identified, ranging in 
mass number from one to two hundred and thirty-eight and in 
atomic number from one to ninety-two. 

1.25. All the statements we have been making are based on 
experimental evidence. The theory of nuclear forces is still 
incomplete, but it has been developed on quantum-mechanical 
principles sufficiently to explain not only the above observations 
but more detailed empirical data on artificial radioactivity and 
on differences between nuclei with odd and even mass numbers. 


1.26. We mentioned the emission of positrons or electrons 
by nuclei seeking stability. Electron emission (beta rays) was 
already familiar in the study of naturally radioactive sub- 
stances, but positron emission was not found in the case of such 
substances. In fact, the general discussion presented above 
obviously was based in part on information that cannot be pre- 
sented in this report. We shall, however, give a brief account of 
the discovery of "artificial" radioactivity and what is now known 
about it. 

1.27. In 1934, Curie and Joliot reported that certain light 
elements (boron, magnesium, aluminum) which had been bom- 
barded with alpha particles continued to emit positrons for some 
time after the bombardment was stopped. In other words, 
alpha-particle bombardment produced radioactive forms of 
boron, magnesium, and aluminum. Curie and Joliot actually 
measured half-lives of 14 minutes, 2.5 minutes, and 3.25 minutes, 
respectively, for the radioactive substances formed by the alpha- 
particle bombardment. 

1.28. This result stimulated similar experiments all over the 
world. In particular, E. Fermi reasoned that neutrons, because 
of their lack of charge, should be effective in penetrating nuclei, 
especially those of high atomic number which repel protons and 
alpha particles strongly. He was able to verify his prediction 
almost immediately, finding that the nucleus of the bombarded 

14 Introduction 

atom captured the neutron and that there was thus produced an 
unstable nucleus which then achieved stability by emitting an 
electron. Thus, the final, stable nucleus was one unit higher in 
mass number and one unit higher in atomic number than the 
initial target nucleus. 

1.29. As a result of innumerable experiments carried out since 
1934, radioactive isotopes of nearly every element in the periodic 
table can now be produced. Some of them revert to stability by 
the emission of positrons, some by the emission of electrons, 
some by a process known as K-electron capture which we shall 
not discuss, and a small number (probably three) by alpha- 
particle emission. Altogether some five hundred unstable nuclear 
species have been observed, and in most cases their atomic 
numbers and mass numbers have been identified. 

1.30. Not only do these artificially radioactive elements play 
an important role throughout the project with which we are 
concerned, but their future value in medicine, in "tracer" 
chemistry, and in many other fields of research can hardly be 



1.31. In describing radioactivity and atomic structure we have 
deliberately avoided quantitative data and have not mentioned 
any applications of the equivalence of mass and energy which we 
announced as the guiding principle of this report. Now when 
we must speak of quantitative details, not merely of general 

1.32. We have spoken of stable and unstable nuclei made up 
of assemblages of protons and neutrons held together by nuclear 
forces. It is a general principle of physics that work must be 
done on a stable system to break it up. Thus, if an assemblage of 
neutrons and protons is stable, energy must be supplied to sepa- 
rate its constituent particles. If energy and mass are really 
equivalent, then the total mass of a stable nucleus should be less 

Introduction 1 5 

than the total mass of the separate protons and neutrons that 
go to make it up. This mass difference, then, should be equivalent 
to the energy required to disrupt the nucleus completely, which 
is called the binding energy. Remember that the masses of all 
nuclei were "approximately" whole numbers. It is the small 
differences from whole numbers that are significant. 

1.33. Consider the alpha particle as an example. It is stable; 
since its mass number is four and its atomic number two it con- 
sists of two protons and two neutrons. The mass of a proton is 
1.00758 and that of a neutron is 1.00893 (see Appendix 2), so 
that the total mass of the separate components of the helium 
nucleus is 

2 X 1.00758 -f 2 X 1.00893 = 4.03302 

whereas the mass of the helium nucleus itself is 4.00280. Neglect- 
ing the last two decimal places we have 4.033 and 4.003, a 
difference of 0.030 mass units. This, then, represents the "binding 
energy" of the protons and neutrons in the helium nucleus. It 
looks small, but recalling Einstein's equation, E = me 2 , we re- 
member that a small amount of mass is equivalent to a large 
amount of energy. Actually 0.030 mass units is equal to 4. 5 X 10~ 5 
ergs per nucleus or 2.7 X 10 19 ergs per gram molecule of helium. 
In units more familiar to the engineer or chemist, this means that 
to break up the nuclei of all the helium atoms in a gram of helium 
would require 1.62 X 10 11 gram calories or 190,000 kilowatt 
hours of energy. Conversely, if free protons and neutrons could 
be assembled into helium nuclei, this energy would be released. 

1.34. Evidently it is worth exploring the possibility of getting 
energy by combining protons and neutrons or by transmuting 
one kind of nucleus into another. Let us begin by reviewing 
present-day knowledge of the binding energies of various nuclei. 


1.35. Chemical atomic-weight determinations give the average 
weight of a large number of atoms of a given element. Unless the 
element has only one isotope, the chemical atomic weight is not 

1 6 Introduction 

proportional to the mass of individual atoms. The mass spectro- 
graph developed by F. W. Aston and others from the earlier 
apparatus of J. J. Thomson measures the masses of individual 
isotopes. Indeed, it was just such measurements that proved the 
existence of isotopes and showed that on the atomic-weight scale 
the masses of all atomic species were very nearly whole numbers. 
These whole numbers, discovered experimentally, are the mass 
numbers which we have already defined and which represent 
the sums of the numbers of the protons and neutrons; their 
discovery contributed largely to our present views that all nuclei 
are combinations of neutrons and protons. 

1.36. Improved mass spectrograph data supplemented in a 
few cases by nuclear reaction data have given accurate figures 
for binding energies for many atomic species over the whole 
range of atomic masses. This binding energy, B, is the difference 
between the true nuclear mass, M, and the sum of the masses 
of all the protons and neutrons in the nucleus. That is, 

B = (ZM P + NM n ) - M 

where M p and M n are the masses of the proton and neutron 
respectively, Z is the number of protons, N = A Z is the num- 
ber of neutrons, and M is the true mass of the nucleus. It is more 
interesting to study the binding energy per particle, B/A, than 
B itself. Such a study shows that, apart from fluctuations in the 
light nuclei, the general trend of the binding energy per particle 
is to increase rapidly to a flat maximum around A = 60 (nickel) 
and then decrease again gradually. Evidently the nuclei in the 
middle of the periodic table nuclei of mass numbers 40 to 100 
are the most strongly bound. Any nuclear reaction where the 
particles in the resultant nuclei are more strongly bound than 
the particles in the initial nuclei will release energy. Speaking in 
thermochemical terms, such reactions are exothermic. Thus, in 
general, energy may be gained by combining light nuclei to 
form heavier ones or by breaking very heavy ones into two or 
three smaller fragments. Also, there are a number of special 
cases of exothermic nuclear disintegrations among the first ten 

Introduction 1 7 

or twelve elements of the periodic table, where the binding 
energy per particle varies irregularly from one element to another. 

1.37. So far we seem to be piling one supposition on another. 
First we assumed that mass and energy were equivalent; now we 
are assuming that atomic nuclei can be rearranged with a 
consequent reduction in their total mass, thereby releasing energy 
which can then be put to use. It is time to talk about some ex- 
periments that convinced physicists of the truth of these statements. 


1.38. As we have already said, Rutherford's work in 1919 on 
artificial nuclear disintegration was followed by many similar 
experiments. Gradual improvement in high-voltage technique 
made it possible to substitute artificially produced high-speed 
ions of hydrogen or helium for natural alpha particles. J. D. 
Cockcroft and E. T. S. Walton in Rutherford's laboratory were 
the first to succeed in producing nuclear changes by such methods. 
In 1932 they bombarded a target of lithium with protons of 700 
kilovolts energy and found that alpha particles were ejected from 
the target as a result of the bombardment. The nuclear reaction 
which occurred can be written symbolically as 

3 Li 7 + iH 1 - 2 He 4 + 2 He 4 

where the subscript represents the positive charge on the nucleus 
(atomic number) and the superscript is the number of massive 
particles in the nucleus (mass number). As in a chemical equa- 
tion, quantities on the left must add up to those on the right; thus 
the subscripts total four and the superscripts eight on each side. 

1.39. Neither mass nor energy has been included in this equa- 
tion. In general, the incident proton and the resultant alpha 
particles will each have kinetic energy. Also, the mass of two 
alpha particles will not be precisely the same as the sum of the 
masses of a proton and a lithium atom. According to our theory, 
the totals of mass and energy taken together should be the same 
before and after the reaction. The masses were known from mass 

1 8 Introduction 

spectra. On the left (Li 7 -f H 1 ) they totalled 8.0241, on the right 
(2He 4 ) 8.0056, so that 0.0185 units of mass had disappeared in 
the reaction. The experimentally determined energies of the 
alpha particles were approximately 8.5 million electron volts 
each, a figure compared to which the kinetic energy of the inci- 
dent proton could be neglected. Thus 0.0185 units of mass had 
disappeared and 17 Mev of kinetic energy had appeared. Now 
0.0185 units of mass is 3.07 X 10~ 26 grams, 17 Mev is 27.2 X 10~ 6 
ergs and c is 3 X 10 10 cm/sec. (See Appendix 2.) If we substitute 
these figures into Einstein's equation, E = me 2 , on the left side 
we have 27.2 X 10~ 6 ergs and on the right side we have 27.6 X 
10~ 6 ergs, so that the equation is found to be satisfied to a good 
approximation. In other words, these experimental results prove 
that the equivalence of mass and energy was correctly stated by 



1.40. Cockcroft and Walton produced protons of fairly high 
energy by ionizing gaseous hydrogen and then accelerating the 
ions in a transformer-rectifier high-voltage apparatus. A similar 
procedure can be used to produce high-energy deuterons from 
deuterium or high-energy alpha particles from helium. Higher 
energies can be attained by accelerating the ions in cyclotrons 
or Van de Graaff machines. However, to obtain high-energy 
gamma radiation or most important of all to obtain neu- 
trons, nuclear reactions themselves must be used as sources. 
Radiations of sufficiently high energy come from certain naturally 
radioactive materials or from certain bombardments. Neutrons 
are commonly produced by the bombardment of certain elements, 
notably beryllium or boron, by natural alpha particles, or by 
bombarding suitable targets with protons or deuterons. The most 
common source of neutrons is a mixture of radium and beryllium 
where the alpha particles from radium and its decay products 
penetrate the Be 9 nuclei, which then give off neutrons and become 

Introduction 1 9 

stable C 12 nuclei (ordinary carbon). A frequently used "beam" 
source of neutrons results from accelerated deuterons impinging 
on "heavy water" ice. Here the high-speed deuterons strike the 
target deuterons to produce neutrons and He 3 nuclei. Half a 
dozen other reactions are also used involving deuterium, lithium, 
beryllium, or boron as targets. Note that in all these reactions the 
total mass number and total charge number are unchanged. 

1.41. To summarize, the agents that are found to initiate 
nuclear reactions are in approximate order of importance 
neutrons, deuterons, protons, alpha particles, gamma rays and, 
rarely, heavier particles. 


1.42. Most atomic nuclei can be penetrated by at least one 
type of atomic projectile (or by gamma radiation). Any such 
penetration may result in a nuclear rearrangement in the course 
of which a fundamental particle is ejected or radiation is emitted 
or both. The resulting nucleus may be one of the naturally avail- 
able stable species, or more likely it may be an atom of a 
different type which is radioactive, eventually changing to still a 
different nucleus. This may in turn be radioactive and, if so, will 
again decay. The process continues until all nuclei have changed 
to a stable type. There are two respects in which these artificially 
radioactive substances differ from the natural ones: many of 
them change by emitting positrons (unknown in natural radio- 
activity) and very few of them emit alpha particles. In every one 
of the cases where accurate measurements have been made, the 
equivalence of mass and energy has been demonstrated and the 
mass-energy total has remained constant. (Sometimes it is neces- 
sary to invoke neutrinos to preserve mass-energy conservation.) 


1.43. A complete description of a nuclear reaction should 
include the nature, mass and energy of the incident particle, also 
the nature (mass number and atomic number), mass and energy 
(usually zero) of the target particle, also the nature, mass and 

20 Introduction 

energy of the ejected particles (or radiation), and finally the 
nature, mass and energy of the remainder. But all of these are 
rarely known and for many purposes their complete specification 
is unnecessary. A nuclear reaction is frequently described by a 
notation that designates first the target by chemical symbol and 
mass number if known, then the projectile, then the emitted 
particle, and then the remainder. In this scheme the neutron 
is represented by the letter n, the proton by p, the deuteron by d, 
the alpha particle by <x, and the gamma ray by 7. Thus the 
radium-beryllium neutron reaction can be written Be 2 (a, n)C 12 
and the deuteron-deuteron reaction H 2 (d, n)He 3 . 


1.44. Considering the five different particles (n, p. d, a, 7) 
both as projectiles and emitted products, we might expect to 
find twenty-five combinations possible. Actually the deuteron 
very rarely occurs as a product particle, and the photon initiates 
only two types of reaction. There are, however, a few other types 
of reaction, such as (n, 2n), (d, H 3 ), and fission, which bring the 
total known types to about twenty-five. Perhaps the (n, 7) reac- 
tion should be specifically mentioned as it is very important in 
one process which will concern us. It is often called "radiative 
capture" since the neutron remains in the nucleus and only a 
gamma ray comes out. 


1.45. So far nothing has been said about the probability of 
nuclear reactions. Actually it varies widely. There is no guarantee 
that a neutron or proton headed straight for a nucleus will 
penetrate it at all. It depends on the nucleus and on the incident 
particle. In nuclear physics, it is found convenient to express 
probability of a particular event by a "cross section." Statistically, 
the centers of the atoms in a thin foil can be considered as points 
evenly distributed over a plane. The center of an atomic projec- 
tile striking this plane has geometrically a definite probability of 

Introduction 21 

passing within a certain distance (r) of one of these points. In fact, 
if there are n atomic centers in an area A of the plane, this prob- 
ability is nrr 2 /A, which is simply the ratio of the aggregate area 
of circles of radius r drawn around the points to the whole area. 
If we think of the atoms as impenetrable steel discs and the 
impinging particle as a bullet of negligible diameter, this ratio 
is the probability that the bullet will strike a steel disc, i.e., that 
the atomic projectile will be stopped by the foil. If it is the fraction 
of impinging atoms getting through the foil which is measured, the 
result can still be expressed in terms of the equivalent stopping 
cross section of the atoms. This notion can be extended to any 
interaction between the impinging particle and the atoms in the 
target. For example, the probability that an alpha particle strik- 
ing a beryllium target will produce a neutron can be expressed 
as the equivalent cross section of beryllium for this type of 

1.46. In nuclear physics it is conventional to consider that the impinging 
particles have negligible diameter. The technical definition of cross section 
for any nuclear process is therefore: 

number of 

processes occurring 
, 7- = (number of target nuclei per cm 2 ) X (nuclear 

incident particles CTOSS scction in cm2) 

It should be noted that this definition is for the cross section per nucleus. 
Gross sections can be computed for any sort of process, such as capture 
scattering, production of neutrons, etc. In many cases, the number of parti- 
cles emitted or scattered in nuclear processes is not measured directly; one 
merely measures the attenuation produced in a parallel beam of incident 
particles by the interposition of a known thickness of a particular material. 
The cross section obtained in this way is called the total cross section and is 
usually denoted by <r. 

1.47. As indicated in paragraph 1.11, the typical nuclear diameter is of 
the order of 10~ 12 cm. We might therefore expect the cross sections for nuclear 
reactions to be of the order of xd 2 /4 or roughly 10~ 24 cm 2 and this is the unit 
in which they are usually expressed. Actually the observed cross sections 
vary enormously. Thus for slow neutrons absorbed by the (n, 7) reaction the 
cross section in some cases is as much as 1,000 X 10~ 24 cm 2 , while the cross 
sections for transmutations by gamma-ray absorption are in the neighbor- 
hood of (1/1,000) X 10~ 24 cm 2 . 

22 Introduction 

IN 1939 


1.48. We have talked glibly about the equivalence of mass 
and energy and about nuclear reactions, such as that of protons 
on lithium, where energy was released in relatively large amounts. 
Now let us ask why atomic power plants did not spring up all 
over the world in the 'thirties. After all, if we can get 2.76 X 10~ 5 
ergs from an atom of lithium struck by a proton, we might expect 
to obtain approximately half a million kilowatt hours by com- 
bining a gram of hydrogen with seven grams of lithium. It looks 
better than burning coal. The difficulties are in producing the 
high-speed protons and in controlling the energy produced. All 
the experiments we have been talking about were done with 
very small quantities of material, large enough in numbers of 
atoms, to be sure, but in terms of ordinary masses infinitesimal 
not tons or pounds or grams, but fractions of micrograms. The 
amount of energy used up in the experiment was always far 
greater than the amount generated by the nuclear reaction. 

1.49. Neutrons are particularly effective in producing nuclear 
disintegration. Why weren't they used? If their initial source was 
an ion beam striking a target, the limitations discussed in the last 
paragraph applied. If a radium and beryllium source was to be 
used, the scarcity of radium was a difficulty. 


1.50. Our common sources of power, other than sunlight and 
water power, are chemical reactions usually the combustion of 
coal or oil. They release energy as the result of rearrangements 
of the outer electronic structures of the atoms, the same kind of 
process that supplies energy to our bodies. Combustion is always 
self-propagating; thus lighting a fire with a match releases enough 
heat to ignite the neighboring fuel, which releases more heat which 
ignites more fuel, and so on. In the nuclear reactions we have 

Introduction 23 

described this is not generally true; neither the energy released 
nor the new particles formed are sufficient to maintain the reac- 
tion. But we can imagine nuclear reactions emitting particles of 
the same sort that initiate them and in sufficient numbers to 
propagate the reaction in neighboring nuclei. Such a self- 
propagating reaction is called a "chain reaction" and such con- 
ditions must be achieved if the energy of the nuclear reactions 
with which we are concerned is to be put to large-scale use. 


1.51. Although there were no atomic power plants built in 
the 'thirties, there were plenty of discoveries in nuclear physics 
and plenty of speculation. A theory was advanced by H. Bethe 
to explain the heat of the sun by a cycle of nuclear changes 
involving carbon, hydrogen, nitrogen, and oxygen, and leading 
eventually to the formation of helium.* This theory is now 
generally accepted. The discovery of a few (n, 2n) nuclear reac- 
tions (i.e., neutron-produced and neutron-producing reactions) 
suggested that a self-multiplying chain reaction might be initiated 
under the right conditions. There was much talk of atomic power 
and some talk of atomic bombs. But the last great step in this 
preliminary period came after four years of stumbling. The 
effects of neutron bombardment of uranium, the most complex 
element known, had been studied by some of the ablest physicists. 
The results were striking but confusing. The story of their gradual 
interpretation is intricate and highly technical, a fascinating tale 
of theory and experiment. Passing by the earlier inadequate 

* The series of reactions postulated was 

(1) eC 1J + iH*-+ 7 Ni 3 

(2) 7 N" -* 6 C 13 + 1C 

(3) 6 C 13 +'iHi-> 7 N" 

(4) TN^ + iH 1 -*^ 15 

(5) 8 15 -> T N + ie 

(6) 7 N 15 + jH l - eG" + 2 He 

The net effect is the transformation of hydrogen into helium and positrons 
(designated as ie) and the release of about thirty million electron volts 

24 Introduction 

explanations, we shall go directly to the final explanation, which, 
as so often happens, is relatively simple. 


1.52. As has already been mentioned, the neutron proved to 
be the most effective particle for inducing nuclear changes. 
This was particularly true for the elements of highest atomic 
number and weight where the large nuclear charge exerts 
strong repulsive forces on deuteron or proton projectiles but not 
on uncharged neutrons. The results of the bombardment of 
uranium by neutrons had proved interesting and puzzling. First 
studied by Fermi and his colleagues in 1934, they were not 
properly interpreted until several years later. 

1.53. On January 16, 1939, Niels Bohr of Copenhagen, Den- 
mark, arrived in this country to spend several months in Prince- 
ton, N. J., and was particularly anxious to discuss some abstract 
problems with Einstein. (Four years later Bohr was to escape 
from Nazi-occupied Denmark in a small boat.) Just before Bohr 
left Denmark two of his colleagues, O. R. Frisch and L. Meitner 
(both refugees from Germany), had told him their guess that the 
absorption of a neutron by a uranium nucleus sometimes caused 
that nucleus to split into approximately equal parts with the 
release of enormous quantities of energy, a process that soon 
began to be called nuclear "fission." The occasion for this 
hypothesis was the important discovery of O. Hahn and F. 
Strassmann in Germany (published in Naturwissenschaften in early 
January 1939) which proved that an isotope of barium was 
produced by neutron bombardment of uranium. Immediately 
on arrival in the United States Bohr communicated this idea to 
his former student J. A. Wheeler and others at Princeton, and 
from them the news spread by word of mouth to neighboring 
physicists including E. Fermi at Columbia University. As a result 
of conversations among Fermi, J. R. Dunning, and G. B. 
Pegram, a search was undertaken at Columbia for the heavy 
pulses of ionization that would be expected from the flying 
fragments of the uranium nucleus. On January 26, 1939, there 

Introduction 25 

was a conference on theoretical physics at Washington, D. C., 
sponsored jointly by the George Washington University and the 
Carnegie Institution of Washington. Fermi left New York to 
attend this meeting before the Columbia fission experiments had 
been tried. At the meeting Bohr and Fermi discussed the problem 
of fission, and in particular Fermi mentioned the possibility that 
neutrons might be emitted during the process. Although this 
was only a guess, its implication of the possibility of a chain 
reaction was obvious. A number of sensational articles were pub- 





lished in the press on this subject. Before the meeting in Washing- 
ton was over, several other experiments to confirm fission had been 
initiated, and positive experimental confirmation was reported 
from four laboratories (Columbia University, Carnegie Institu- 
tion of Washington, Johns Hopkins University, University of 
California) in the February 15, 1939, issue of the Physical Review. 
By this time Bohr had heard that similar experiments had been 
made in his laboratory in Copenhagen about January 15. (Letter 
by Frisch to Nature dated January 16, 1939, and appearing in the 
February 18 issue.) F. Joliot in Paris had also published his first 
results in the Comptes Rendus of January 30, 1939. From this time 
on there was a steady flow of papers on the subject of fission, so 
that by the time (December 6. 1939) L. A. Turner of Princeton 

26 Introduction 

wrote a review article on the subject in the Reviews of Modern 
Physics nearly one hundred papers had appeared. Complete 
analysis and discussion of these papers have appeared in Turner's 
article and elsewhere. 


1 .54. Consider the suggestion of Frisch and Meitner in the light 
of the two general trends that had been discovered in nuclear 
structure: first, that the proportion of neutrons goes up with 
atomic number; second, that the binding energy per particle 
is a maximum for the nuclei of intermediate atomic number. 
Suppose the U-238 nucleus is broken exactly in half; then, 
neglecting the mass of the incident neutron, we have two nuclei 
of atomic number 46 and mass number 119. But the heaviest 
stable isotope of palladium (Z = 46) has a mass number of only 
110. Therefore to reach stability each of these imaginary new 
nuclei must eject nine neutrons, becoming 4ePd 110 nuclei; or 
four neutrons in each nucleus must convert themselves to protons 
by emitting electrons, thereby forming stable tin nuclei of mass 
number 119 and atomic number 50; or a combination of such 
ejections and conversions must occur to give some other pair of 
stable nuclei. Actually, as was suggested by Hahn and Strass- 
mann's identification of barium (Z = 56, A = 135 to 140) as a 
product of fission, the split occurs in such a way as to produce 
two unequal parts of mass numbers about 140 and 90 with the 
emission of a few neutrons and subsequent radioactive decay by 
electron emission until stable nuclei are formed. Calculations 
from binding-energy data show that any such rearrangement 
gives an aggregate resulting mass considerably less than the 
initial mass of the uranium nucleus, and thus that a great deal 
of energy must be released. 

1.55. Evidently, there were three major implications of the 
phenomenon of fission: the release of energy, the production of 
radioactive atomic species and the possibility of a neutron chain 
reaction. The energy release might reveal itself in kinetic energy 
of the fission fragments and in the subsequent radioactive dis- 

Introduction 27 

integration of the products. The possibility of a neutron chain 
reaction depended on whether neutrons were in fact emitted 
a possibility which required investigation. 

1.56. These were the problems suggested by the discovery of 
fission, the kind of problem reported in the journals in 1939 and 
1940 and since then investigated largely in secret. The study of 
the fission process itself, including production of neutrons and 
fast fragments, has been largely carried out by physicists using 
counters, cloud chambers, etc. The study and identification of 
the fission products has been carried out largely by chemists, 
who have had to perform chemical separations rapidly even with 
submicroscopic quantities of material and to make repeated 
determinations of the half-lives of unstable isotopes. We shall 
summarize the state of knowledge as of June 1 940. By that time 
the principal facts about fission had been discovered and revealed 
to the scientific world. A chain reaction had not been obtained, 
but its possibility at least in principle was clear and several 
paths that might lead to it had been suggested. 



1.57. All the following information was generally known in 
June 1940, both here and abroad: 

(1) That three elements uranium, thorium, and protoactinium 
when bombarded by neutrons sometimes split into approxi- 
mately equal fragments, and that these fragments were isotopes 
of elements in the middle of the periodic table, ranging from 
selenium (Z = 34) to lanthanum (Z = 57). 

(2) That most of these fission fragments were unstable, decaying 
radioactively by successive emission of beta particles through a 
series of elements to various stable forms. 

(3) That these fission fragments had very great kinetic energy. 

(4) That fission of thorium and protoactinum was caused only 

28 Introduction 

by fast neutrons (velocities of the order of thousands of miles 
per second). 

(5) That fission in uranium could be produced by fast or slow 
(so-called thermal velocity) neutrons; specifically, that thermal 
neutrons caused fission in one isotope, U-235, but not in the 
other, U-238, and that fast neutrons had a lower probability of 
causing fission in U-235 than thermal neutrons. 

(6) That at certain neutron speeds there was a large capture 
cross section in U-238 producing U-239 but not fission. 

(7) That the energy released per fission of a uranium nucleus 
was approximately 200 million electron volts. 

(8) That high-speed neutrons were emitted in the process of 

(9) That the average number of neutrons released per fission 
was somewhere between one and three. 

(10) That high-speed neutrons could lose energy by inelastic 
collision with uranium nuclei without any nuclear reaction 
taking place. 

(11) That most of this information was consistent with the semi- 
empirical theory of nuclear structure worked out by Bohr and 
Wheeler and others; this suggested that predictions based on 
this theory had a fair chance of success. 


1.58. It was realized that radiative capture of neutrons by 
U-238 would probably lead by two successive beta-ray emissions 
to the formation of a nucleus for which Z = 94 and A = 239. 
Consideration of the Bohr- Wheeler theory of fission and of certain 
empirical relations among the nuclei by L. A. Turner and others 
suggested that this nucleus would be a fairly stable alpha emitter 
and would probably undergo fission when bombarded by thermal 
neutrons. Later the importance of such thermal fission to the 
maintenance of the chain reaction was foreshadowed in private 
correspondence and discussion. In terms of our present knowledge 
and notation the particular reaction suggested is as follows: 

Introduction 29 

where Np and Pu are the chemical symbols now used for the 
two new elements, neptunium and plutonium; on 1 represents the 
neutron, and _ie represents an ordinary (negative) electron. 
Plutonium 239 is the nucleus rightly guessed to be fissionable by 
thermal neutrons. It will be discussed fully in later chapters. 


1.59. By 1940 nuclear reactions had been intensively studied 
for over ten years. Several books and review articles on nuclear 
physics had been published. New techniques had been developed 
for producing and controlling nuclear projectiles, for studying 
artificial radioactivity, and for separating submicroscopic quan- 
tities of chemical elements produced by nuclear reactions. Isotope 
masses had been measured accurately. Neutron-capture cross 
sections had been measured. Methods of slowing down neutrons 
had been developed. Physiological effects of neutrons had been 
observed; they had even been tried in the treatment of cancer. 
All such information was generally available; but it was very 
incomplete. There were many gaps and many inaccuracies. 
The techniques were difficult and the quantities of materials 
available were often submicroscopic. Although the fundamental 
principles were clear, the theory was full of unverified assump- 
tions and calculations were hard to make. Predictions made in 
1940 by different physicists of equally high ability were often 
at variance. The subject was in all too many respects an art, 
rather than a science. 


1.60. Looking back on the year 1940, we see that all the pre- 
requisites to a serious attack on the problem of producing atomic 
bombs and controlling atomic power were at hand. It had been 
proved that mass and energy were equivalent. It had been proved 
that the neutrons initiating fission of uranium reproduced them- 

30 Introduction 

selves in the process and that therefore a multiplying chain 
reaction might occur with explosive force. To be sure, no one 
knew whether the required conditions could be achieved, but 
many scientists had clear ideas as to the problems involved and 
the directions in which solutions might be sought. The next 
chapter of this report gives a statement of the problems and 
serves as a guide to the developments of the past five years. 



2.1. From the time of the first discovery of the large amounts 
of energy released in nuclear reactions to the time of the discovery 
of uranium fission, the idea of atomic power or even atomic bombs 
was discussed off and on in scientific circles. The discovery of 
fission made this talk seem much less speculative, but realization 
of atomic power still seemed in the distant future and there was 
an instinctive feeling among many scientists that it might not, 
in fact, ever be realized. During 1939 and 1940 many public 
statements, some of them by responsible scientists, called atten- 
tion to the enormous energy available in uranium for explosives 
and for controlled power, so that U-235 became a familiar by- 
word indicating great things to come. The possible military 
importance of uranium fission was called to the attention of the 
government (see Chapter III), and in a conference with repre- 
sentatives of the Navy Department in March 1939 Fermi sug- 
gested the possibility of achieving a controllable reaction using 
slow neutrons or a reaction of an explosive character using fast 
neutrons. He pointed out, however, that the data then available 
might be insufficient for accurate predictions. 

2.2. By the summer of 1940 it was possible to formulate the 
problem fairly clearly, although it was still far from possible to 
answer the various questions involved or even to decide whether 
a chain reaction ever could be obtained. In this chapter we shall 
give a statement of the problem in its entirety. For purposes of 
clarification we may make use of some knowledge which actually 
was not acquired until a later date. 


2.3. The principle of operation of an atomic bomb or power 
plant utilizing uranium fission is simple enough. If one neutron 


32 Statement of the Problem 

causes a fission that produces more than one new neutron, the 
number of fissions may increase tremendously with the release 
of enormous amounts of energy. It is a question of probabilities. 
Neutrons produced in the fission process may escape entirely 
from the uranium, may be captured by uranium in a process not 
resulting in fission, or may be captured by an impurity. Thus the 
question of whether a chain reaction does or does not go depends 
on the result of a competition among four processes: 

(1) escape, 

(2) non-fission capture by uranium, 

(3) non-fission capture by impurities, 

(4) fission capture. 

If the loss of neutrons by the first three processes is less than the 
surplus produced by the fourth, the chain reaction occurs; other- 
wise it does not. Evidently any one of the first three processes 
may have such a high probability in a given arrangement that 
the extra neutrons created by fission will be insufficient to keep 
the reaction going. For example, should it turn out that process 
(2) non-fission capture by uranium has a much higher prob- 
ability than fission capture, there would presumably be no 
possibility of achieving a chain reaction. 

2.4. An additional complication is that natural uranium con- 
tains three isotopes: U-234, U-235, and U-238, present to the 
extent of approximately 0.006, 0.7, and 99.3 percent, respectively. 
We have already seen that the probabilities of processes (2) 
and (4) are different for different isotopes. We have also seen 
that the probabilities are different for neutrons of different 

2.5. We shall now consider the limitations imposed by the 
first three processes and how their effects can be minimized. 


2.6. The relative number of neutrons which escape from a 
quantity of uranium can be minimized by changing the size 

Statement of the Problem 33 

and shape. In a sphere any surface effect is proportional to the 
square of the radius, and any volume effect is proportional to the 
cube of the radius. Now the escape of neutrons from a quantity 
of uranium is a surface effect depending on the area of the surface, 
but fission capture occurs throughout the material and is there- 
fore a volume effect. Consequently the greater the amount of 
uranium, the less probable it is that neutron escape will pre- 
dominate over fission capture and prevent a chain reaction. 
Loss of neutrons by non-fission capture is a volume effect like 
neutron production by fission capture, so that increase in size 
makes no change in its relative importance. 

2.7. The critical size of a device containing uranium is defined 
as the size for which the production of free neutrons by fission 
is just equal to their loss by escape and by non-fission capture. 
In other words, if the size is smaller than critical, then by 
definition no chain reaction will sustain itself. In principle it 
was possible in 1940 to calculate the critical size, but in practice 
the uncertainty of the constants involved was so great that the 
various estimates differed widely. It seemed not improbable that 
the critical size might be too large for practical purposes. Even 
now estimates for untried arrangements vary somewhat from 
time to time as new information becomes available. 


2.8. In Chapter I we said that thermal neutrons have the 
highest probability of producing fission of U-235 but we also 
said that the neutrons emitted in the process of fission had high 
speeds. Evidently it was an oversimplification to say that the 
chain reaction might maintain itself if more neutrons were 
created by fission than were absorbed. For the probability both 
of fission capture and of non-fission capture depends on the speed 
of the neutrons. Unfortunately, the speed at which non-fission 
capture is most probable is intermediate between the average 
speed of neutrons emitted in the fission process and the speed 
at which fission capture is most probable. 

34 Statement of the Problem 

2.9. For some years before the discovery of fission, the cus- 
tomary way of slowing down neutrons was to cause them to 
pass through material of low atomic weight, such as hydrogenous 
material. The process of slowing down or moderation is simply 
one of elastic collisions between high-speed particles and particles 
practically at rest. The more nearly identical the masses of 
neutron and struck particle the greater the loss of kinetic energy 
by the neutron. Therefore the light elements are most effective 
as "moderators," i.e., slowing down agents, for neutrons. 

2.10. It occurred to a number of physicists that it might be 
possible to mix uranium with a moderator in such a way that 
the high-speed fission neutrons, after being ejected from uranium 
and before re-encountering uranium nuclei, would have their 
speeds reduced below the speeds for which non-fission capture 
is highly probable. Evidently the characteristics of a good moder- 
ator are that it should be of low atomic weight and that it should 
have little or no tendency to absorb neutrons. Lithium and boron 
are excluded on the latter count. Helium is difficult to use because 
it is a gas and forms no compounds. The choice of moderator 
therefore lay among hydrogen, deuterium, beryllium, and 
carbon. Even now no one of these substances can be excluded 
from the list of practical possibilities. It was E. Fermi and L. 
Szilard who proposed the use of graphite as a moderator for a 
chain reaction. 


2.11. The general scheme of using a moderator mixed with 
the uranium was pretty obvious. A specific manner of using a 
moderator was first suggested in this country, so far as we can 
discover, by Fermi and Szilard. The idea was to use lumps of 
uranium of considerable size imbedded in a matrix of moderator 
material. Such a lattice can be shown to have real advantages 
over a homogeneous mixture. As the constants were more accu- 
rately determined, it became possible to calculate theoretically 
the type of lattice that would be most effective. 



















/ PA 

ST\ < 











/ I \ 




Statement of the Problem 35 


2.12. In Chapter I it was stated that for neutrons of certain 
intermediate speeds (corresponding to energies of a few electron 
volts) U-238 has a large capture cross section for the production 
of U-239 but not for fission. There is also a considerable prob- 
ability of inelastic (i.e., non-capture-producing) collisions between 
high-speed neutrons and U-238 nuclei. Thus the presence of the 
U-238 tends both to reduce the speed of the fast neutrons and to 
effect the capture of those of moderate speed. Although there 
may be some non-fission capture by U-235, it is evident that 
if we can separate the U-235 from the U-238 and discard the 
U-238, we can reduce non-fission capture and can thus promote 
the chain reaction. In fact, the probability of fission of U-235 
by high-speed neutrons may be great enough to make the use 
of a moderator unnecessary once the U-238 has been removed. 
Unfortunately, U-235 is present in natural uranium only to the 
extent of about one part in 140. Also, the relatively small 
difference in mass between the two isotopes makes separation 
difficult. In fact, in 1940 no large-scale separation of isotopes 
had ever been achieved except for hydrogen, whose two isotopes 
differ in mass by a factor of two. Nevertheless, the possibility of 
separating U-235 was recognized early as being of the greatest 
importance, and such separation has, in fact, been one of the 
two major lines of Project effort during the past five years. 


2.13. It has been stated above that the cross section for capture 
of neutrons varies greatly among different materials. In some it 
is very high compared to the maximum fission cross section of 
uranium. If, then, we are to hope to achieve a chain reaction, 
we must reduce effect (3) non-fission capture by impurities 
to the point where it is not serious. This means very careful 
purification of the uranium metal and very careful purification 
of the moderator. Calculations show that the maximum per- 

36 Statement of the Problem 

missible concentrations of many impurity elements are a few 
parts per million in either the uranium or the moderator. When 
it is recalled that up to 1 940 the total amount of uranium metal 
produced in this country was not more than a few grams and 
even this was of doubtful purity, that the total amount of metallic 
beryllium produced in this country was not more than a few 
pounds, that the total amount of concentrated deuterium pro- 
duced was not more than a few pounds, and that carbon had 
never been produced in quantity with anything like the purity 
required of a moderator, it is clear that the problem of producing 
and purifying materials was a major one. 


2.14. The problems that have been discussed so far have to 
do merely with the realization of the chain reaction. If such a 
reaction is going to be of use, we must be able to control it. The 
problem of control is different depending on whether we are 
interested in steady production of power or in an explosion. In 
general, the steady production of atomic power requires a slow- 
neutron-induced fission chain reaction occurring in a mixture 
or lattice of uranium and moderator, while an atomic bomb 
requires a fast-neutron-induced fission chain reaction in U-235 
or Pu-239, although both slow- and fast-neutron fission may con- 
tribute in each case. It seemed likely, even in 1940, that by using 
neutron absorbers a power chain reaction could be controlled. 
It was also considered likely, though not certain, that such a chain 
reaction would be self-limiting by virtue of the lower probability 
of fission-producing capture when a higher temperature was 
reached. Nevertheless, there was a possibility that a chain-reacting 
system might get out of control, and it therefore seemed necessary 
to perform the chain-reaction experiment in an uninhabited 


2.15. Up to this point we have been discussing how to produce 
and control a nuclear chain reaction but not how to make use 

Statement of the Problem 37 

of it. The technological gap between producing a controlled 
chain reaction and using it as a large-scale power source or an 
explosive is comparable to the gap between the discovery of fire 
and the manufacture of a steam locomotive. 

2.16. Although production of power has never been the 
principal object of this project, enough attention has been given 
to the matter to reveal the major difficulty: the attainment of 
high- temperature operation. An effective heat engine must not 
only develop heat but must develop heat at a high temperature. 
To run a chain-reacting system at a high temperature and to 
convert the heat generated to useful work is very much more 
difficult than to run a chain-reacting system at a low temperature. 

2.17. Of course, the proof that a chain reaction is possible does 
not itself insure that nuclear energy can be effective in a bomb. 
To have an effective explosion it is necessary that the chain 
reaction build up extremely rapidly; otherwise only a small 
amount of the nuclear energy will be utilized before the bomb 
flies apart and the reaction stops. It is also necessary that no 
premature explosion occur. This entire "detonation" problem 
was and still remains one of the most difficult problems in de- 
signing a high-efficiency atomic bomb. 


2.18. So far, all our discussion has been primarily concerned 
with the use of uranium itself. We have already mentioned the 
suggestion that the element of atomic number 94 and mass 239, 
commonly referred to as plutonium, might be very effective. 
Actually, we now believe it to be of value comparable to pure 
U-235. We have mentioned the difficulty of separating U-235 
from the more abundant isotope U-238. These two isotopes are, 
of course, chemically identical. But plutonium, although pro- 
duced from U-238, is a different chemical element. Therefore, 
if a process could be worked out for converting some of the U-238 
to plutonium, a chemical separation of the plutonium from ura- 
nium might prove more practicable than the isotopic separation 
of U-235 from U-238. 

38 Statement of the Problem 

2.19. Suppose that we have set up a controllable chain reac- 
tion in a lattice of natural uranium and a moderator say carbon, 
in the form of graphite. Then as the chain reaction proceeds, 
neutrons are emitted in the process of fission of the U-235 and 
many of these neutrons are absorbed by U-238. This produces 
U-239, each atom of which then emits a beta particle, becoming 
neptunium (osNp 239 ). Neptunium, in turn, emits another beta 
particle, becoming plutonium ( 9 4Pu 239 ), which emits an alpha 
particle, decaying again to U-235, but so slowly that in effect 
it is a stable element. (See figure on p. 8.) If, after the reaction 
has been allowed to proceed for a considerable time, the mixture 
of metals is removed, it may be possible to extract the plutonium 
by chemical methods and purify it for use in a subsequent 
fission chain reaction of an explosive nature. 


2.20. Three ways of increasing the likelihood of a chain 
reaction have been mentioned: use of a moderator; attainment 
of high purity of materials; use of special material, either U-235 
or Pu. The three procedures are not mutually exclusive, and 
many schemes have been proposed for using small amounts of 
separated U-235 or Pu-239 in a lattice composed primarily of 
ordinary uranium or uranium oxide and of a moderator or two 
different moderators. Such proposed arrangements are usually 
called "enriched piles." 


2.21. All our previous discussion has centered on the direct or 
indirect use of uranium, but it was known that both thorium 
and protoactinium also underwent fission when bombarded by 
high-speed neutrons. The great advantage of uranium, at least 
for preliminary work, was its susceptibility to slow neutrons. 
There was not very much consideration given to the other two 
substances. Protoactinium can be eliminated because of its 

Statement of the Problem 39 

scarcity in nature. Thorium is relatively plentiful but has no 
apparent advantage over uranium. 

2.22. It is not to be forgotten that theoretically many nuclear 
reactions might be used to release energy. At present we see no 
way of initiating or controlling reactions other than those 
involving fission, but some such synthesis as has already been 
mentioned as a source of solar energy may eventually be pro- 
duced in the laboratory. 


2.23. Obviously it was impossible in the summer of 1940 to 
make more than guesses as to what amounts of materials would 
be needed to produce: 

(1) a chain reaction with use of a moderator: 

(2) a chain-reaction bomb in pure, or at least enriched, U-235 
or plutonium. 

A figure of one to one hundred kilograms of U-235 was com- 
monly given at this time for the critical size of a bomb. This 
would, of course, have to be separated from at least 140 times 
as much natural uranium. For a slow-neutron chain reaction 
using a moderator and unseparated uranium it was almost 
certain that tons of metal and of moderator would be required. 


2.24. Estimates of the composition of the earth's crust show 
uranium and thorium both present in considerable quantities 
(about 4 parts per million of uranium and 12 parts per million 
of thorium in the earth's crust). Deposits of uranium ore are 
known to exist in Colorado, in the Great Bear Lake region of 
northern Canada, in Joachimstal in Czechoslovakia, and in the 
Belgian Congo. Many other deposits of uranium ore are known, 
but their extent is in many cases unexplored. Uranium is always 
found with radium although in much larger quantity. Both are 
often found with vanadium ores. Small quantities of uranium 
oxide have been used for many years in the ceramics industry. 

2.25. Thorium is also rather widely distributed, occurring as 

40 Statement of the Problem 

thorium oxide in fairly high concentration in monazite sands. 
Such sands are found to some extent in this country but particu- 
larly in Brazil and in British India. 

2.26. Early rough estimates, which are probably optimistic, 
were that the nuclear energy available in known deposits of 
uranium was adequate to supply the total power needs of this 
country for 200 years (assuming utilization of U-238 as well as 

2.27. As has already been mentioned, little or no uranium 
metal had been produced up to 1940 and information was so 
scant that even the melting point was not known. (For example, 
the Handbook of Physics and Chemistry for 1 943-1 944 says only that 
the melting point is below 1850 C. whereas we now know it 
to be in the neighborhood of 1150.) Evidently, as far as uranium 
was concerned, there was no insurmountable difficulty as regards 
obtaining raw materials or producing the metal, but there were 
very grave questions as to how long it would take and how much 
it would cost to produce the necessary quantities of pure metal. 

2.28. Of the materials mentioned above as being suitable for 
moderators, deuterium had the most obvious advantages. It is 
present in ordinary hydrogen to the extent of about one part in 
5,000. By 1940 a number of different methods for separating it 
from hydrogen had been developed, and a few liters had been 
produced in this country for experimental purposes. The only 
large-scale production had been in a Norwegian plant, from 
which several hundred liters of heavy water (DzO, deuterium 
oxide) had come. As in the case of uranium, the problem was 
one of cost and time. 

2.29. Beryllium in the form of beryllium silicates is widely 
found but only in small quantities of ore. Its use as an alloying 
agent has become general in the last few years; for such use, 
however, it is not necessary to produce the beryllium in metallic 
form. In 1940 only 700 pounds of the metal were produced in 
this country. 

2.30. As far as carbon was concerned, the situation was 
obviously quite different. There were many hundreds of tons 

Statement of the Problem 41 

of graphite produced every year in this country. This was one 
of the reasons why graphite looked very desirable as a moderator. 
The difficulties lay in obtaining sufficient quantities of graphite 
of the required purity, particularly in view of the expanding 
needs of war industry. 


2.31. Requirements of time and money depended not only 
on many unknown scientific and technological factors but also 
on policy decisions. Evidently years of time and millions of dollars 
might be required to achieve the ultimate objective. About all 
that was attempted at this time was the making of estimates as 
to how long it would take and how much it would cost to clarify 
the scientific and technological prospects. It looked as if it would 
not be a very great undertaking to carry along the development 
of the thermal-neutron chain reaction in a graphite-uranium 
lattice to the point of finding out whether the reaction would 
in fact go. Estimates made at the time were that approximately 
a year and $100,000 would be required to get an answer. These 
estimates applied to a chain-reacting system of very low power 
without a cooling system or any means for using the energy 


2.32. It had been known for a long time that radioactive 
materials were dangerous. They give off very penetrating radia- 
tions gamma rays which are much like X-rays in their phy- 
siological effects. They also give off beta and alpha rays which, 
although less penetrating, can still be dangerous. The amounts 
of radium used in hospitals and in ordinary physical measure- 
ments usually comprise but a few milligrams. The amounts of 
radioactive material produced by the fission of uranium in a 
relatively small chain-reacting system may be equivalent to 
hundreds or thousands of grams of radium. A chain-reacting 
system also gives off intense neutron radiation known to be 

42 Statement of the Problem 

comparable to gamma rays as regards health hazards. Quite 
apart from its radioactive properties, uranium is poisonous 
chemically. Thus, nearly all work in this field is hazardous 
particularly work on chain reactions and the resulting radio- 
active products. 


2.33. There were two ways of attacking the problem. One was 
to conduct elaborate series of accurate physical measurements on 
absorption cross sections of various materials for various neutron- 
induced processes and various neutron energies. Once such data 
were available, calculations as to what might be done in the 
way of a chain reaction could be made with fair accuracy. The 
other approach was the purely empirical one of mixing uranium 
or uranium compounds in various ways with various moderators 
and observing what happened. Similar extremes of method were 
possible in the case of the isotope-separation problem. Actually 
an intermediate or compromise approach was adopted in both 


2.34. The expected military advantages of uranium bombs 
were far more spectacular than those of a uranium power plant. 
It was conceivable that a few uranium bombs might be decisive 
in winning the war for the side first putting them into use. Such 
thoughts were very much in the minds of those working in this 
field, but the attainment of a slow-neutron chain reaction 
seemed a necessary preliminary step in the development of our 
knowledge and became the first objective of the group interested 
in the problem. This also seemed an important step in con- 
vincing military authorities and the more skeptical scientists 
that the whole notion was not a pipe dream. Partly for these 
reasons and partly because of the extreme secrecy imposed about 
this time, the idea of an atomic bomb does not appear much in 
the records between the summer of 1940 and the fall of 1941. 

Statement of the Problem 43 


2.35. If all the atoms in a kilogram of U-235 undergo fission, 
the energy released is equivalent to the energy released in the 
explosion of about 20,000 short tons of TNT. If the critical size 
of a bomb turns out to be practical say, in the range of one to 
one hundred kilograms and all the other problems can be 
solved, there remain two questions. First, how large a percentage 
of the fissionable nuclei can be made to undergo fission before 
the reaction stops; i.e., what is the efficiency of the explosion? 
Second, what is the effect of so concentrated a release of energy? 
Even if only 1 per cent of the theoretically available energy is 
released, the explosion will still be of a totally different order of 
magnitude from that produced by any previously known type 
of bomb. The value of such a bomb was thus a question for 
military experts to consider very carefully. 


2.36. It had been established (1) that uranium fission did 
occur with release of great amounts of energy; and (2) that in 
the process extra neutrons were set free which might start a 
chain reaction. It was not contrary to any known principle that 
such a reaction should take place and that it should have very 
important military application as a bomb. However, the idea 
was revolutionary and therefore suspect; it was certain that 
many technical operations of great difficulty would have to be 
worked out before such a bomb could be produced. Probably 
the only materials satisfactory for a bomb were either U-235, 
which would have to be separated from the 140- times more 
abundant isotope U-238, or Pu-239, an isotope of the hitherto 
unknown element plutonium, which would have to be generated 
by a controlled chain-reacting process itself hitherto unknown. 
To achieve such a controlled chain reaction it was clear that 
uranium metal and heavy water or beryllium or carbon might 
have to be produced in great quantity with high purity. Once 
bomb material was produced a process would have to be devel- 

44 Statement of the Problem 

oped for using it safely and effectively. In some of the processes, 
health hazards of a new kind would be encountered. 


2.37. By the summer of 1940 the National Defense Research 
Committee had been formed and was asking many of the scien- 
tists in the country to work on various urgent military problems. 
Scientific personnel was limited although this was not fully 
realized at the time. It was, therefore, really difficult to decide 
at what rate work should be carried forward on an atomic 
bomb. The decision had to be reviewed at frequent intervals 
during the subsequent four years. An account of how these policy 
decisions were made is given in Chapters III and V. 



3.1. The announcement of the hypothesis of fission and its 
experimental confirmation took place in January 1939, as has 
already been recounted in Chapter I. There was immediate 
interest in the possible military use of the large amounts of energy 
released in fission. At that time American-born nuclear physicists 
were so unaccustomed to the idea of using their science for mili- 
tary purposes that they hardly realized what needed to be done. 
Consequently the early efforts both at restricting publication 
and at getting government support were stimulated largely by 
a small group of foreign-born physicists centering on L. Szilard 
and including E. Wigner, E. Teller, V. F. Weisskopf, and E. 

S'. In the spring of 1939 the group mentioned above enlisted 
Niels Bohr's cooperation in an attempt to stop publication of 
further data by voluntary agreement. Leading American and 
British physicists agreed, but F. Joliot, France's foremost nuclear 
physicist, refused, apparently because of the publication of one 
letter in the Physical Review sent in before all Americans had been 
brought into the agreement. Consequently publication con- 
tinued freely for about another year although a few papers were 
withheld voluntarily by their authors. 

3.3. At the April 1940 meeting of the Division of Physical 
Sciences of the National Research Council, G. Breit proposed 
formation of a censorship committee to control publication in 
all American scientific journals. Although the reason for this 


46 Administrative History to 1941 

suggestion was primarily the desire to control publication of 
papers on uranium fission, the "Reference Committee" as finally 
set up a little later that spring (in the National Research Coun- 
cil) was a general one, and was organized to control publication 
policy in all fields of possible military interest. The chairman of 
the committee was L. P. Eisenhart; other members were G. Breit, 
W. M. Clark, H. Fletcher, E. B. Fred, G. B. Pegram, H. C. Urey, 
L. H. Weed, and E. G. Wever. Various subcommittees were 
appointed, the first one of which had to do with uranium fission. 
G. Breit served as chairman of this subcommittee; its other 
members were J. W. Beams, L. J. Briggs, G. B. Pegram, H. C. 
Urey, and E. Wigner. In general, the procedure followed was to 
have the editors of various journals send copies of papers in this 
field, in cases where the advisability of publication was in doubt, 
either directly to Breit or indirectly to him through Eisenhart. 
Breit then usually circulated them to all members of the sub- 
committee for consideration as to whether or not they should be 
published^ and informed the editors as to the outcome. This 
arrangement was very successful in preventing publication and 
was still nominally in effect, in modified form, in June 1945. 
Actually the absorption of most physicists in this country into 
war work of one sort of another soon reduced the number of 
papers referred to the committee practically to the vanishing 
point. It is of interest to note that this whole arrangement was a 
purely voluntary one; the scientists of the country are to be 
congratulated on their complete cooperation. It is to be hoped 
that it will be possible after the war to publish these papers at 
least in part so that their authors may receive proper professional 
credit for their contributions. 


3.4. On the positive side government interest and support of 
research in nuclear physics the history is a much more com- 
plicated one. The first contact with the government was made by 
Pegram of Columbia in March 1939. Pegram telephoned to the 

Administrative History to 1941 47 

Navy Department and arranged for a conference between 
representatives of the Navy Department and Fermi. The only 
outcome of this conference was that the Navy expressed interest 
and asked to be kept informed. The next attempt to interest the 
government was stimulated by Szilard and Wigner. In July 
1939 they conferred with A. Einstein, and a little later Einstein, 
Wigner, and Sziiard discussed the problem with Alexander 
Sachs of New York. In the fall Sachs, supported by a letter from 
Einstein, explained to President Roosevelt the desirability of 
encouraging work in this field. The President appointed a com- 
mittee, known as the "Advisory Committee on Uranium" and 
consisting of Briggs (director of the Bureau of Standards) as chair- 
man, Colonel K. F. Adamson of the Army Ordnance Department, 
and Commander G. C. Hoover of the Navy Bureau of Ordnance, 
and requested this committee to look into the problem. This was 
the only committee on uranium that had official status up to the 
time of organization of the National Defense Research Committee 
in June 1940. The committee met very informally and included 
various additional scientific representatives in its meetings. 

3.5. The first meeting of the Uranium Committee was on 
October 21, 1939 and included, besides the committee members, 
F. L. Mohler, Alexander Sachs, L. Szilard, E. Wigner, E. Teller, 
and R. B. Roberts. The result of this meeting was a report dated 
November 1, 1939, and transmitted to President Roosevelt by 
Briggs, Adamson, and Hoover. This report made eight recom- 
mendations, which need not be enumerated in detail. It is 
interesting, however, that it specifically mentions both atomic 
power and an atomic bomb as possibilities. It specifically recom- 
mended procurement of 4 tons of graphite and 50 tons of uranium 
oxide for measurements of the absorption cross section of carbon. 
Others of the recommendations either were of a general nature 
or were never carried out. Apparently a memorandum prepared 
by Szilard was more or less the basis of the discussion at this 

3.6. The first transfer of funds ($6,000) from the Army and 
Navy to purchase materials in accordance with the recommenda- 

48 Administrative History to 1941 

tion of November 1 is reported in a memorandum from Briggs 
to General E. M. Watson (President Roosevelt's aide) on 
February 20, 1940. The next meeting of the "Advisory Com- 
mittee on Uranium" was on April 28, 1940 and was attended by 
Sachs, Wigner, Pegram, Fermi, Szilard, Briggs, Admiral H. G. 
Bowen, Colonel Adamson, and Commander Hoover. By the 
time of this meeting two important new factors had come into 
the picture. First, it had been discovered that the uranium fission 
caused by neutrons of thermal velocities occurred in the U-235 
isotope only. Second, it had been reported that a large section 
of the Kaiser Wilhelm Institute in Berlin had been set aside for 
research on uranium. Although the general tenor of the discus- 
sion at this meeting seems to have been that the work should be 
pushed more vigorously, no definite recommendations were 
made. It was pointed out that the critical measurements on 
carbon already under way at Columbia should soon give a result, 
and the implication was that definite recommendations should 
wait for such a result. 

3.7. Within the next few weeks a number of people concerned, 
particularly Sachs, urged the importance of greater support and 
of better organization. Their hand was strengthened by the 
Columbia results (as reported, for example, in a letter from Sachs 
to General Watson on May 15, 1940) showing that the carbon 
absorption was appreciably lower than had been previously 
thought and that the probability of carbon being satisfactory 
as a moderator was therefore considerable. Sachs was also 
active in looking into the question of ore supply. On June 1, 1940, 
Sachs, Briggs, and Urey met with Admiral Bowen to discuss 
approaching officials of the Union Miniere of the Belgian Congo. 
Such an approach was made shortly afterwards by Sachs. 

3.8. The general status of the problem was discussed by a 
special advisory group called together by Briggs at the National 
Bureau of Standards on June 15, 1940. This meeting was attended 
by Briggs, Urey, M. A. Tuve, Wigner, Breit, Fermi, Szilard, 
and Pegram. "After full discussion, the recommendation of the 
group to the Uranium Committee was that funds should be 

Administrative History to 1941 49 

sought to support research on the uranium-carbon experiment 
along two lines: 

(A) further measurements of the nuclear constants involved 
in the proposed type of reaction ; 

(B) experiments with amounts of uranium and carbon equal 
to about one fifth to one quarter of the amount that could 
be estimated as the minimum in which a chain reaction 
would sustain itself. 

"It was estimated that about $40,000 would be necessary for 
further measurements of the fundamental constants and that 
approximately $100,000 worth of metallic uranium and pure 
graphite would be needed for the intermediate experiment." 
(Quotations from memorandum of Pegram to Briggs, dated 
August 14, 1940.) 


3.9. Before any decisions made at this meeting could be put 
into effect, the organization of the National Defense Research 
Committee was announced in June 1 940, and President Roosevelt 
gave instructions that the Uranium Committee should be recon- 
stituted as a subcommittee of the NDRC, reporting to Vannevar 
Bush (chairman, NDRC). The membership of this recon- 
stituted Uranium Committee was as follows: Briggs, Chairman; 
Pegram, Urey, Beams, Tuve, R. Gunn and Breit. On authoriza- 
tion from Briggs, Breit consulted Wigner and Teller frequently 
although they were not members of the committee. From that 
time until the summer of 1941 this committee continued in 
control with approximately the same membership. Its recom- 
mendations were transmitted by Briggs to the NDRC, and suit- 
able contracts were made between the NDRC and various 
research institutions. The funds, however, were first supplied by 
the Army and Navy, not from regular NDRC appropriations. 


3.10. The first contract let under this new set-up was to 
Columbia University for the two lines of work recommended 

50 Administrative History to 1941 

at the June 15 meeting as described above. The project was 
approved by the NDRC and the first NDRC contract (NDCrc-32) 
was signed November 8, 1940, being effective from November 1, 
1940, to November 1, 1941. The amount of this contract was 

3.11. Only very small expenditures had been made before 
the contract went into effect. For example, about $3,200 had 
been spent on graphite and cadmium, this having been taken 
from the $6,000 allotted by the Army and Navy in February, 

3.12. We shall not attempt to review in detail the other con- 
tracts that were arranged prior to December 1941. Their number 
and total amount grew gradually. Urey began to work on isotope 
separation by the centrifuge method under a Navy contract in 
the fall of 1940. Other contracts were granted to Columbia 
University, Princeton University, Standard Oil Development 
Company, Cornell University, Carnegie Institution of Wash- 
ington, University of Minnesota, Iowa State College, Johns 
Hopkins University, National Bureau of Standards, University 
of Virginia, University of Chicago, and University of California 
in the course of the winter and spring of 1940-1941 until by 
November 1941 the total number of projects approved was 
sixteen, totalling about $300,000. 

3.13. Scale of expenditure is at least a rough index of activity. 
It is therefore interesting to compare this figure with those in 
other branches of war research. By November 1941 the total 
budget approved by NDRC for the Radiation Laboratory at 
the Massachusetts Institute of Technology was several million 
dollars. Even a relatively small project like that of Section S of 
Division A of the NDRC had spent or been authorized to spend 
$136,000 on work that proved valuable but was obviously not 
potentially of comparable importance to the uranium work. 


3.14. The Uranium Committee as formed in the summer of 
1940 continued substantially unchanged until the summer of 

Administrative History to 1941 51 

1941. At that time the main committee was somewhat enlarged 
and subcommittees formed on isotope separation, theoretical 
aspects, power production and heavy water.* It was thereafter 
called the Uranium Section or the S-l Section of NDRC. 
Though not formally disbanded until the summer of 1942, this 
revised committee was largely superseded in December 1941 
(see Chapter V). 


3.15. In the spring of 1941, Briggs, feeling that an impartial 
review of the problem was desirable, requested Bush to appoint 
a reviewing committee. Bush then formally requested F. B. 
Jewett, president of the National Academy of Sciences, to 
appoint such a committee. Jewett complied, appointing A. H. 
Compton, chairman; W. D. Coolidge, E. O. Lawrence, J. C. 
Slater, J. H. Van Vleck, and B. Gherardi. (Because of illness, 
Gherardi was unable to serve.) This committee was instructed to 
evaluate the military importance of the uranium problem and 
to recommend the level of expenditure at which the problem 
should be investigated. 

3.16. This committee met in May and submitted a report. 
(This report and the subsequent ones will be summarized in the 
next chapter.) On the basis of this report and the oral exposition 
by Briggs before a meeting of the NDRC, an appropriation of 
$267,000 was approved by the NDRC at its meeting of July 18, 
1941, and the probability that much larger expenditures would 
be necessary was indicated. Bush asked for a second report with 
emphasis on engineering aspects, and in order to meet this 
request O. E. Buckley of the Bell Telephone Laboratories and 

* Uranium Section: Briggs, chairman; Pegram, vice-chairman; S. K. 
Allison, Beams, Breit, E. U. Condon, H. D. Smyth, Urey. 

Separation Subsection: Urey, chairman; Beams. 

Power Production Subsection: Pegram, chairman; Allison, Fermi, 
Smyth, Szilard. 

Heavy Water Subsection: Urey, chairman; T. H. Chilton. 

Theoretical Aspects Subsection: Fermi, chairman; Breit, C. H. Eckart, 
Smyth, Szilard, J. A. Wheeler. 

52 Administrative History to 1941 

L. W. Chubb of the Westinghouse Electrical and Manufacturing 
Company were added to the committee. (Compton was in South 
America during the summer and therefore did not participate in 
the summer meetings of the committee.) The second report was 
submitted by Coolidge. As a result of new measurements of the 
fission cross section of U-235 and of increasing conviction that 
isotope separation was possible, in September 1941, Compton 
and Lawrence suggested to J. B. Conant of NDRC, who was 
working closely with Bush, that a third report was desirable. Since 
Bush and Conant had learned during the summer of 1941 that 
the British also felt increasingly optimistic, the committee was 
asked to make another study of the whole subject. For this pur- 
pose the committee was enlarged by the addition of W. K. Lewis, 
R. S. Mulliken, and G. B. Kistiakowsky. This third report was 
submitted by Compton on November 6, 1941. 


3.17. Beginning in 1940 there was some interchange of infor- 
mation with the British and during the summer of 1941 Bush 
learned that they had been reviewing the whole subject in the 
period from April to July. They too had been interested in 
the possibility of using plutonium; in fact, a suggestion as to the 
advisability of investigating plutonium was contained in a letter 
from J. D. Cockcroft to R. H. Fowler dated December 28, 1940. 
Fowler, who was at that time acting as British scientific liaison 
officer in Washington, passed Cockcroft's letter on to Lawrence. 
The British never pursued the plutonium possibility, since they 
felt their limited manpower should concentrate on U-235. 
Chadwick, at least, was convinced that a U-235 bomb of great 
destructive power could be made, and the whole British group 
felt that the separation of U-235 by diffusion was probably feasible. 

3.18. Accounts of British opinion, including the first draft of 
the British report reviewing the subject, were made available to 
Bush and Conant informally during the summer of 1941, although 
the official British report of July 1 5 was first transmitted to Conant 
by G. P. Thomson on October 3. Since, however, the British 

Administrative History to 1941 53 

review was not made available to the committee of the National 
Academy of Sciences, the reports by the Academy committee 
and the British reports constituted independent evaluations of the 
prospects of producing atomic bombs. 

3.19. Besides the official and semi-official conferences, there 
were many less formal discussions held, one of these being stimu- 
lated by M. L. E. Oliphant of England during his visit to this 
country in the summer of 1 941 . As an example of such informal 
discussion we might mention talks among Conant, Compton, 
and Lawrence at the University of Chicago semicentennial cele- 
bration in September 1941. The general conclusion was that the 
program should be pushed; and this conclusion in various forms 
was communicated to Bush by a number of persons. 

3.20. In the fall of 1 941 Urey and Pegram were sent to England 
to get first-hand information on what was being done there. 
This was the first time that any Americans had been to England 
specifically in connection with the uranium problem. The report 
prepared by Urey and Pegram confirmed and extended the 
information that had been received previously. 


3.21. As a result of the reports prepared by the National 
Academy committee, by the British, and by Urey and Pegram, 
and of the general urging by a number of physicists, Bush, as 
Director of the Office of Scientific Research and Development 
(of which NDRG is a part), decided that the uranium work 
should be pushed more aggressively. 

3.22. Before the National Academy issued its third report and 
before Pegram and Urey visited England, Bush had taken up 
the whole uranium question with President Roosevelt and Vice- 
President Wallace. He summarized for them the British views, 
which were on the whole optimistic, and pointed out the un- 
certainties of the predictions. The President agreed that it was 
desirable to broaden the program, to provide a different organiza- 
tion, to provide funds from a special source, and to effect com- 
plete interchange of information with the British. It was agreed 

54 Administrative History to 1941 

to confine discussions of general policy to the following group: 
The President, Vice-President, Secretary of War, Chief of Staff, 
Bush, and Conant. This group was often referred to as the Top 
Policy Group. 

3.23. By the time of submission of the National Academy's 
third report and the return of Urey and Pegram from England, 
the general plan of the reorganization was beginning to emerge. 
The Academy's report was more conservative than the British 
report, as Bush pointed out in his letter of November 27, 1941, to 
President Roosevelt. It was, however, sufficiently optimistic to 
give additional support to the plan of enlarging the work. The 
proposed reorganization was announced at a meeting of the 
Uranium Section just before the Pearl Harbor attack and will be 
described in Chapter V. 


3.24. In March 1939, only a few weeks after the discovery of 
uranium fission, the possible military importance of fission was 
called to the attention of the government. In the autumn of 1939 
the first government committee on uranium was created. In the 
spring of 1 940 a mechanism was set up for restricting publication 
of significant articles in this field. When the NDRC was set up 
in June 1940, the Uranium Committee was reconstituted under 
the NDRC. However, up to the autumn of 1941 total expendi- 
tures were relatively small. In December 1941, after receipt of 
the National Academy report and information from the British, 
the decision was made to enlarge and reorganize the program. 



4.1. In Chapter II the general problems involved in producing 
a chain reaction for military purposes were described. Early in 
the summer of 1 940 the questions of most immediate importance 

(1) Could any circumstances be found under which the chain 
reaction would go? 

(2) Could the isotope U-235 be separated on a large scale? 

(3) Could moderator and other materials be obtained in 
sufficient purity and quantity? 

Although there were many subsidiary problems, as will appear 
in the account of the progress made in the succeeding eighteen 
months, these three questions determined the course of the work. 



4.2. In June 1940, nearly all work on the chain reaction was 
concentrated at Columbia under the general leadership of 
Pegram, with Fermi and Szilard in immediate charge. It had 
been concluded that the most easily produced chain reaction 
was probably that depending on thermal neutron fission in a 
heterogeneous mixture of graphite and uranium. In the spring 
of 1940 Fermi, Szilard and H. L. Anderson had improved the 
accuracy of measurements of the capture cross section of carbon 
for neutrons, of the resonance (intermediate-speed) absorption 
of neutrons by U-238, and of the slowing down of neutrons in 


56 Progress to December 1941 

4.3. Pegram, in a memorandum to Briggs on August 14, 1940, 
wrote, "It is not very easy to measure , these quantities with 
accuracy without the use of large quantities of material. The 
net results of these experiments in the spring of 1940 were that 
the possibility of the chain reaction was not definitely proven, 
while it was still further from being definitely disproven. On 
the whole, the indications were more favorable than any con- 
clusions that could fairly have been claimed from previous 

4.4. At a meeting on June 15 (see Chapter III) these results 
were discussed and it was recommended that (A) further measure- 
ments be made on nuclear constants, and (B) experiments be 
made on lattices of uranium and carbon containing amounts of 
uranium from one fifth to one quarter the estimated critical 


4.5. Pegram's report of February 15, 1941 shows that most 
of the work done up to that time was on (A), while (B), the 
so-called intermediate experiment, was delayed by lack of 

4.6. Paraphrasing Pegram's report, the main progress was as 

(a) The slowing down of neutrons in graphite was investigated by 
studying the intensity of activation of various detectors (rhodium, 
indium, iodine) placed at various positions inside a rectangular 
graphite column of dimensions 3X3X8 feet when a source of 
neutrons was placed therein. By suitable choice of cadmium 
screens the effects of resonance* and thermal neutrons were 
investigated separately, f A mathematical analysis, based on dif- 

* See footnote p. 58. 

f The presence of neutrons can be detected by ionization chambers or 
counters or by the artificial radioactivity induced in various metal foils. 
(See Appendix 1.) The response of each of these detectors depends on the 
particular characteristics of the detector and on the speed of the neutrons 
(e.g., neutrons of about 1.5 volts energy are particularly effective in acti- 
vating indium). Furthermore, certain materials have very large absorption 

(Progress to December 1941 57 

fusion theory, of the experimental data made it possible to predict 
the results to be expected in various other arrangements. These 
results, coupled with theoretical studies of the diffusion of thermal 
neutrons, laid a basis for future calculations of the number of 
thermal and resonance neutrons to be found at any point in a 
graphite mass of given shape when a given neutron source is 
placed at a specified position within or near the graphite. , 

(b) The number of neutrons emitted in fission. The experiments on 
slowing down neutrons showed that high-energy (high-speed) 
neutrons such as those from fission were practically all reduced 
to thermal energies (low speeds) after passing through 40 cm or 
more of graphite. A piece of uranium placed in a region where 
thermal neutrons are present absorbs the thermal neutrons and 
as fission occurs re-emits fast neutrons, which are easily- 
distinguished from the thermal neutrons. By a series of measure- 
ments with and without uranium present and with various 
detectors and absorbers, it is possible to get a value for the con- 
stant 77, the number of neutrons emitted per thermal neutron 
absorbed by uranium. This is not the number of neutrons emitted 
per fission, but is somewhat smaller than that number since not 
every absorption causes fission. 

(c) Lattice theory. Extensive calculations were made on the 
probable number of neutrons escaping from lattices of various 
designs and sizes. This was fundamental for the so-called inter- 
mediate experiment, mentioned above as item (B). 


4.7. Early in 1941 interest in the general chain-reaction 
problem by individuals at Princeton, Chicago, and California 
led to the approval of certain projects at those institutions. 
Thereafter the work of these groups was coordinated with the 
work at Columbia, forming parts of a single large program. 

cross sections for neutrons of particular ranges of speed (e.g., cadmium for 
thermal neutrons). Thus measurements with different detectors with or 
without various absorbers give some indication of both the number of 
neutrons present and their energy distribution. However, the state of the art 
of such measurements is rather crude. 

58 Progress to December 1941 


4.8. In Chapter II it is stated that there were advantages in a 
lattice structure or "pile" with uranium concentrated in lumps 
regularly distributed in a matrix of moderator. This was the 
system on which the Columbia group was working. As is so often 
the case, the fundamental idea is a simple one. If the uranium 
and the moderator are mixed homogeneously, the neutrons on 
the average will lose energy in small steps between passages 
through the uranium so that in the course of their reduction to 
thermal velocity the chance of their passing through uranium at 
any given velocity, e.g., at a velocity corresponding to resonance 
absorption, is great. But, if the uranium is in large lumps spaced 
at large intervals in the moderator, the amounts of energy lost by 
neutrons between passages from one lump of uranium to another 
will be large and the chance of their reaching a uranium lump 
with energy just equal to the energy of resonance absorption is 
relatively small. Thus the chance of absorption by U-238 to 
produce U-239, compared to the chance of absorption as thermal 
neutrons to cause fission, may be reduced sufficiently to allow a 
chain reaction to take place. If one knew the exact values of the 
cross sections of each uranium isotope for each type of absorption 
and every range of neutron speed, and had similar knowledge for 
the moderator, one could calculate the "optimum lattice," i.e., 
the best size, shape and spacing for the lumps of uranium in the 
matrix of moderator. Since such data were only partially known, 
a direct experimental approach appeared to be in order. Conse- 
quently it was proposed that the absorption of neutrons by 
uranium should be measured under conditions similar to those ex- 
pected in a chain-reacting pile employing graphite as moderator. 

4.9. Experiments of this type were initiated at Columbia, and 
were continued at Princeton in February 1941. Essentially the 

* The term "resonance absorption" is used to describe the very strong 
absorption of neutrons by U-238 when the neutron energies are in certain 
definite portions of the energy region from to 1,000 electron volts. Such 
resonance absorption demonstrates the existence of nuclear energy levels at 
corresponding energies. On some occasions the term resonance absorption 
is used to refer to the whole energy region in the neighborhood of such levels. 

Progress to December 1941 59 

experiment consisted of studying the absorption of neutrons in 
the energy range extending from a few thousand electron volts 
down to a fraction of an electron volt (thermal energies), the 
absorption taking place in different layers of uranium or uranium 
oxide spheres embedded in a pile of graphite. 

4.10. In these experiments, a source of neutrons was provided 
by a mean of protons (accelerated by a cyclotron) impinging on 
a beryllium target. (The resulting yield of neutrons was equiva- 
lent to the yield from a radium-beryllium source of about 3,500 
curies strength.) The neutrons thus produced had a wide, con- 
tinuous, velocity distribution. They proceeded from this source 
into a large block of graphite. By placing the various uranium or 
uranium-oxide spheres inside the graphite block at various 
positions representing increasing distances from the source, 
absorption of neutrons of decreasing average speeds down to 
thermal speeds was studied. It was found that the total absorption 
of neutrons by such spheres could be expressed in terms of a 
"surface" effect and a "mass" effect. 

4.11. These experiments, involving a variety of sphere sizes, 
densities, and positions were continued until the spring of 1942, 
when most of the group was moved to Chicago. Similar experi- 
ments performed at a later date at the University of Indiana by 
A. C. G. Mitchell and his co-workers have verified and in some 
cases corrected the Princeton data, but the Princeton data were 
sufficiently accurate by the summer of 1941 to be used in plan- 
ning the intermediate-pile experiments and the subsequent ex- 
periments on operating piles. 

4.12. The experimental work on resonance absorption at 
Princeton was done by R. R. Wilson, E. C. Creutz, and their 
collaborators, under the general leadership of H. D. Smyth; they 
benefited from the constant help of Wigner and Wheeler and 
frequent conferences with the Columbia group. 


4.13. About July 1941 the first lattice structure of graphite and 
uranium was set up at Columbia. It was a graphite cube about 

60 Progress to December 1941 

8 feet on an edge, and contained about 7 tons of uranium oxide 
in iron containers distributed at equal intervals throughout the 
graphite. A preliminary set of measurements was made on this 
structure in August 1941. Similar structures of somewhat larger 
size were set up and investigated during September and October, 
and the so-called exponential method (described below) of 
determining the multiplication factor was developed and first 
applied. This work was done by Fermi and his assistants, H. L. 
Anderson, B. Feld, G. Weil, and W. H. Zinn. 

4.14. The multiplication-factor experiment is rather similar 
to that already outlined for the determination of 17, the number 
of neutrons produced per thermal neutron absorbed. A radium- 
beryllium neutron source is placed near the bottom of the lattice 
structure and the number of neutrons is measured at various 
points throughout the lattice. These numbers are then compared 
with the corresponding numbers determined when no uranium 
is present in the graphite mass. Evidently the absorption of 
neutrons by U-238 to produce U-239 tends to reduce the number 
of neutrons, while the fissions tend to increase the number. 
The question is: Which predominates? or, more precisely, Does 
the fission production of neutrons predominate over all neutron- 
removal processes other than escape? Interpretation of the experi- 
mental data on this crucial question involves many corrections, 
calculations, and approximations, but all reduce in the end to a 
single number, the multiplication factor k. 


4.15. The whole success or failure of the uranium project 
depended on the multiplication factor k, sometimes called the 
reproduction factor. If k could be made greater than 1 in a 
practical system, the project would succeed; if not, the chain 
reaction would never be more than a dream. This is clear from 
the following discussion, which applies to any system containing 
fissionable material. Suppose that there is a certain number of 
free neutrons present in the system at a given time. Some of 
these neutrons will themselves initiate fissions and will thus 

Progress to December 1941 61 

directly produce new neutrons. The multiplication factor k is the 
ratio of the number of these new neutrons to the number of free 
neutrons originally present. Thus, if in a given pile comprising 
uranium, carbon, impurities, containers, etc., 100 neutrons are 
produced by fission, some will escape, some will be absorbed in 
the uranium without causing fission, some will be absorbed in 
the carbon, in the containers or in impurities, and some will 
cause fission, thereby producing more neutrons.* If the fissions 
are sufficiently numerous and sufficiently effective individually, 
more than 100 new neutrons will be produced and the system is 
chain reacting. If the number of new neutrons is 105, k = 1.05. 
But if the number of new neutrons per 100 initial ones is 99, 
k = .99 and no chain reaction can maintain itself. 

4.16. Recognizing that the intermediate or "exponential" 
experiment described above was too small to be chain reacting, 
we see that it was a matter of great interest whether any larger 
pile of the same lattice structure would be chain reacting. This 
could be determined by calculating what the value of k would be 
for an infinitely large lattice of this same type. In other words, 
the problem was to calculate what the value of k would be if no 
neutrons leaked away through the sides of the pile. Actually it is 
found that, once a chain-reacting system is well above the critical 
size say two or three times as great and is surrounded by what 
is called a reflector, the effective value of k differs very little from 
that for infinite size provided that k is near 1.00. Consequently, 
it has become customary to characterize the chain-reaction 
potentialities of different mixtures of metal and moderator by 
the value of k* the multiplication constant obtained by assuming 
infinite size of pile. 

4.17. The value of k* as reported by Fermi to the Uranium 
Section in the fall of 1941 was about 0.87. This was based on 
results from the second Columbia intermediate experiment. All 
agreed that the multiplication factor could be increased by greater 
purity of materials, different lattice arrangements, etc. None 
could say with certainty that it could be made greater than one. 

* See drawing facing p. 35. 

62 Progress to December 1941 


4.18. At about the same time that the work on resonance 
absorption was started at Princeton, S. K. Allison, at the sugges- 
tion of A. H. Compton, began work at Chicago under a contract 
running from January 1, 1941 to August 1, 1941. The stated 
objectives of the work were to investigate (a) the increase in 
neutron production when the pile is enclosed in a beryllium 
envelope or "reflector," and (b) the cross sections of beryllium. 
A new contract was authorized on July 18, 1941, to run to June 
30, 1942. This stated the somewhat broader objective of investi- 
gating uranium-beryllium-carbon systems generally. The appro- 
priations involved were modest: $9,500 for the first contract, and 
$30,000 for the second contract. 

4.19. As has already been pointed out in Chapter II, beryllium 
has desirable qualities as a moderator because of its low atomic 
weight and low neutron-absorption cross section; there was also 
the possibility that a contribution to the number of neutrons 
would be realized from the (n, 2n) reaction in beryllium. The 
value of the cross section was not precisely known; furthermore 
it was far from certain that any large amount of pure beryllium 
could be obtained. Allison's problem was essentially similar to the 
Columbia problem, except for the use of beryllium in place of 
graphite. Because of the scarcity of beryllium it was suggested 
that it might be used in conjunction with graphite or some other 
moderator, possibly as a reflector. 

4.20. In the Chicago experiments, neutrons produced with the 
aid of a cyclotron were caused to enter a pile of graphite and 
beryllium. Allison made a number of measurements on the 
slowing down and absorption by graphite which were valuable 
checks on similar experiments at Columbia. He finally was able 
to obtain enough beryllium to make significant measurements 
which showed that beryllium was a possible moderator com- 
parable to graphite. However, beryllium was not in fact used at 
all extensively in view of the great difficulty of producing it in 
quantity in the required structural forms. 

Progress to December 1941 63 

4.21. This Chicago project as described above became part 
of the Metallurgical Laboratory project established at the Uni- 
versity of Chicago early in 1942. 


4.22. Both the intermediate experiments at Columbia and the 
continued resonance-absorption work at Princeton required 
skilful theoretical interpretation. Fermi worked out the theory 
of the "exponential" pile and Wigner the theory of resonance 
absorption; both these men were constantly conferring and 
contributing to many problems. Wheeler of Princeton, Breit of 
Wisconsin, and Eckart of Chicago to mention only a few also 
made contributions to general pile theory and related topics. 
Altogether one can say that by the end of 1941 the general theory 
of the chain reaction for slow neutrons was almost completely 
understood. It was the numerical constants and technological 
possibilities that were still uncertain. 

4.23. On the theory of a fast-neutron reaction in U-235 a good 
deal of progress had also been made. In particular, new estimates 
of the critical size were made, and it was predicted that possibly 
10 per cent of the total energy might be released explosively. 
On this basis one kilogram of U-235 would be equivalent to 
2,000 tons of TNT. The conclusions are reviewed below in con- 
nection with the National Academy Report. It is to be remem- 
bered that there are two factors involved: (1) how large a fraction 
of the available fission energy will be released before the reaction 
stops; (2) how destructive such a highly concentrated explosion 
will be. 


4.24. In Chapter I mention is made of the suggestion that the 
element 94, later christened plutonium, would be formed by 
beta-ray disintegrations of U-239 resulting from neutron absorp- 
tion by U-238 and that plutonium would probably be an alpha- 
particle emitter of long half-life and would undergo fission when 
bombarded by neutrons. In the summer of 1940 the nuclear 

64 Progress to December 1941 

physics group at the University of California in Berkeley was 
urged to use neutrons from its powerful cyclotron for the pro- 
duction of plutonium, and to separate it from uranium and 
investigate its fission properties. Various pertinent experiments 
were performed by E. Se'gre, G. T. Seaborg, J. W. Kennedy, 
and A. C. Wahl at Berkeley prior to 1941 and were reported by 
E. O. Lawrence to the National Academy Committee (see below) 
in May 1941 and also in a memorandum that was incorporated 
in the Committee's second report dated July 11, 1941. It will be 
seen that this memorandum includes one important idea not 
specifically emphasized by others (paragraph 1.58), namely, 
the production of large quantities of plutonium for use in a bomb. 

4.25. We quote from Lawrence's memorandum as follows: 
"Since the first report of the National Academy of Sciences Com- 
mittee on Atomic Fission, an extremely important new possibility 
has been opened for the exploitation of the chain reaction with 
unseparated isotopes of uranium. Experiments in the Radiation 
Laboratory of the University of California have indicated (a) that 
element 94 is formed as a result of capture of a neutron by 
uranium 238 followed by two successive beta-transformations, 
and furthermore (b) that this transuranic element undergoes 
slow neutron fission and therefore presumably behaves like 
uranium 235. 

"It appears accordingly that, if a chain reaction with unsepa- 
rated isotopes is achieved, it may be allowed to proceed violently 
for a period of time for the express purpose of manufacturing 
element 94 in substantial amounts. This material could be ex- 
tracted by ordinary chemistry and would presumably be the 
equivalent of uranium 235 for chain reaction purposes. 

"If this is so, the following three outstanding important possi- 
bilities are opened: 

"1. Uranium 238 would be available for energy production, 
thus increasing about one hundred fold the total atomic energy 
obtainable from a given quantity of uranium. 

"2. Using element 94 one may envisage preparation of small 
chain reaction units for power purposes weighing perhaps a 

Progress to December 1941 65 

hundred pounds instead of a hundred tons as probably would be 
necessary for units using natural uranium. 

"3. If large amounts of element 94 were available it is likely 
that a chain reaction with fast neutrons could be produced. In 
such a reaction the energy would be released at an explosive 
rate which might be described as 'super bomb.' " 


4.26. As previously stated, the fragments resulting from fission 
are in most cases unstable nuclei, that is, artificially radioactive 
materials. It is common knowledge that the radiations from 
radioactive materials have deadly effects akin to the effects of 

4.27. In a chain-reacting pile these radioactive fission products 
build up as the reaction proceeds. (They have, in practice, turned 
out to be the most troublesome feature of a reacting pile.) Since 
they differ chemically from the uranium, it should be possible 
to extract them and use them like a particularly vicious form of 
poison gas. This idea was mentioned in the National Academy 
report (see paragraph 4.48) and was developed in a report 
written December 10, 1941, by E. Wigner and H. D. Smyth, 
who concluded that the fission products produced in one day's 
run of a 100,000 kw chain-reacting pile might be sufficient to 
make a large area uninhabitable. 

4.28. Wigner and Smyth did not recommend the use of radio- 
active poisons nor has such use been seriously proposed since 
by the responsible authorities, but serious consideration was 
given to the possibility that the Germans might make surprise 
use of radioactive poisons, and accordingly defensive measures 
were planned. 



4.29. In Chapter I the attribution of thermal-neutron fission 
of uranium to the U-235 isotope was mentioned as being experi- 

66 Progress to December 1941 

mentally established. This was done by partly separating minute 
quantities of the uranium isotopes in A. O. Nier's mass specto- 
graph and then studying the nuclear properties of the samples. 
Additional small samples were furnished by Nier in the summer 
of 1941 and studied by N. P. Heydenburg and others at M. A. 
Tuve's laboratory at the Department of Terrestrial Magnetism 
of the Carnegie Institution of Washington. But results of such 
experiments were still preliminary, and it was evident that 
further study of larger and more completely separated samples 
was desirable. 

4.30. The need of larger samples of U-235 stimulated E. O. 
Lawrence at Berkeley to work on electromagnetic separation. 
He was remarkably successful and by December 6, 1941 reported 
that he could deposit in one hour one microgram of U-235 from 
which a large proportion of the U-238 had been removed. 

4.31. Previously, at a meeting of the Uranium Committee, 
Smyth of Princeton had raised the question of possible large-scale 
separation of isotopes by electromagnetic means but had been 
told that it had been investigated and was considered impossible. 
Nevertheless, Smyth and Lawrence at a chance meeting in 
October 1941 discussed the problem and agreed that it might yet 
be possible. Smyth again raised the question at a meeting of the 
Uranium Committee on December 6 and at the next meeting 
(December 18, 1941) there was a general discussion of large-scale 
electromagnetic methods in connection with Lawrence's report 
of his results already mentioned. The consequences of this dis- 
cussion are reported in Chapter XI. 


4.32. Though we have made it clear that the separation of 
U-235 from U-238 might be fundamental to the whole success of 
the project, little has been said about work in this field. Such work 
had been going on since the summer of 1940 under the general 
direction of H. C. Urey at Columbia. Since this part of the ura- 
nium work was not very much affected by the reorganization in 

Progress to December 1941 67 

December 1941, a detailed account of the work is reserved for 
Chapters IX and X. Only a summary is presented here. 

4.33. After careful review and a considerable amount of 
experimenting on other methods, it had been concluded that the 
two most promising methods of separating large quantities of 
U-235 from U-238 were by the use of centrifuges and by the use 
of diffusion through porous barriers. In the centrifuge, the forces 
acting on the two isotopes are slightly different because of their 
differences in mass. In the diffusion through barriers, the rates of 
diffusion are slightly different for the two isotopes, again because 
of their differences in mass. Each method required the uranium to 
be in gaseous form, which was an immediate and serious limita- 
tion since the only suitable gaseous compound of uranium then 
known was uranium hexafluoride. In each method the amount 
of enrichment to be expected in a single production unit or "stage" 
was very small; this indicated that many successive stages would 
be necessary if a high degree of enrichment was to be attained. 

4.34. By the end of 1941 each method had been experimentally 
demonstrated in principle; that is, single-stage separators had 
effected the enrichment of the U-235 on a laboratory scale to 
about the degree predicted theoretically. K. Cohen of Columbia 
and others had developed the theory for the single units and for 
the series or "cascade" of units that would be needed. Thus it 
was possible to estimate that about 5,000 stages would be neces- 
sary for one type of diffusion system and that a total area of 
many acres of diffusion barrier would be required in a plant 
separating a kilogram of U-235 each day. Corresponding cost 
estimates were tens of millions of dollars. For the centrifuge the 
number of stages would be smaller, but it was predicted that a 
similar production by centrifuges would require 22,000 separately 
driven, extremely high-speed centrifuges, each three feet in length 
at a comparable cost. 

4.35. Of course, the cost estimates could not be made accu- 
rately since the technological problems were almost completely 
unsolved, but these estimates as to size and cost of plant did serve 
to emphasize the magnitude of the undertaking. 

68 Progress to December 1941 


4.36. In September 1940, P. H. Abelson submitted to Briggs a 
17-page memorandum suggesting the possibility of separating 
the isotopes of uranium by thermal diffusion in liquid uranium 
hexafluoride. R. Gunn of the Naval Research Laboratory was 
also much interested in the uranium problem and was appointed 
a member of the Uranium Committee when it was reorganized 
under the NDRC in the summer of 1940. As a result of Abelson's 
suggestion and Gunn's interest, work was started on thermal dif- 
fusion at the National Bureau of Standards. This work was 
financed by funds from the Navy Department and in 1940 was 
transferred to the Naval Research Laboratory, still under the 
direction of Abelson, where it was continued. 

4.37. We shall discuss the thermal-diffusion work further in a 
later chapter, but we may mention here that significant results 
had already been obtained by the end of 1941 and that in Janu- 
ary 1942, using a single separation column, a separation factor 
had been obtained which was comparable or superior to the one 
obtained up to that time in preliminary tests on the diffusion 
and centrifuge methods. 


4.38. It was pointed out in Chapter II that deuterium ap- 
peared very promising as a moderator because of its low absorp- 
tion and good slowing-down property but unpromising because 
of its scarcity. Interest in a deuterium moderator was stimulated 
by experimental results obtained in Berkeley demonstrating that 
the deuterium absorption cross section for neutrons was, in fact, 
almost zero. Since oxygen has a very low absorption coefficient 
for neutrons, it was usually assumed that the deuterium would be 
used combined with oxygen, that is, in the very convenient 
material: heavy water. Work at Columbia on possible methods 
of large-scale concentration of heavy water was initiated in 
February 1941 under the direction of H. C. Urey (under an 
OSRD contract). Early in 1941, R. H. Fowler of England re- 

Progress to December 1941 69 

ported the interest of the British group in a moderator of deu- 
terium in the form of heavy water and their conviction that a 
chain reaction would go in relatively small units of uranium and 
heavy water. 

4.39. Urey and A. von Grosse had already been considering 
the concentration of heavy water by means of a catalytic ex- 
change reaction between hydrogen gas and liquid water. This 
process depends on the fact that, when isotopic equilibrium is 
established between hydrogen gas and water, the water contains 
from three to four times as great a concentration of deuterium as 
does the hydrogen gas. During 1941, this exchange reaction 
between water and hydrogen was investigated at Columbia and 
in the Frick Chemical Laboratory at Princeton and extensive 
work was done toward developing large-scale methods of pro- 
ducing materials suitable for catalyzing the reaction. 

4.40. The further development of this work and of other 
methods of producing heavy water are discussed in Chapter IX. 
Like the other isotope-separation work at Columbia, this work 
was relatively unaffected by the reorganization in December 
1941. It is mentioned in preliminary fashion here to indicate 
that all the principal lines of approach were under investigation 
in 1941. 


4.41. By the end of 1941 not very much progress had been 
made in the production of materials for use in a chair-reacting 
system. The National Bureau of Standards and the Columbia 
group were in contact with the Metal Hydrides Company of 
Beverly, Massachusetts. This company was producing some 
uranium in powdered form, but efforts to increase its production 
and to melt the powdered metal into solid ingots had not been 
very successful. 

4.42. Similarly, no satisfactory arrangement had been made 
for obtaining large amounts of highly purified graphite. The 
graphite in use at Columbia had been obtained from the U. S. 
Graphite Company of Saginaw, Michigan. It was of high purity 

70 Progress to December 1941 

for a commercial product, but it did contain about one part in 
500,000 of boron, which was undesirable. 

4.43. Largely through the interest of Allison the possibility of 
increasing the production of beryllium had been investigated to 
the extent of ascertaining that it would be difficult and expensive, 
but probably possible. 

4.44. Though little progress had been made on procurement, 
much progress had been made on analysis. The development of 
sufficiently accurate methods of chemical analysis of the materials 
used has been a problem of the first magnitude throughout the 
history of the project, although sometimes overshadowed by the 
more spectacular problems encountered. During this period 
C. J. Rodden and others at the National Bureau of Standards 
were principally responsible for analyses; H. T. Beans of Columbia 
also cooperated. By 1942 several other groups had started analyt- 
ical sections which have been continuously active ever since. 

4.45. To summarize, by the end of 1941 there was no evidence 
that procurement of materials in sufficient quantity and purity 
was impossible, but the problems were far from solved. 


4.46. Prior to the autumn of 1941 there had been some ex- 
change of reports with the British and some discussion with 
British scientific representatives who were here on other business. 
In September 1941, it was decided that Pegram and Urey should 
get first-hand information by a trip to England. They completed 
their trip in the first week of December 1941. 

4.47. In general, work in England had been following much 
the same lines as in this country. As to the chain-reaction problem, 
their attention had focussed on heavy water as a moderator 
rather than graphite; as to isotope separation, they had done ex- 
tensive work on the diffusion process including the general theory 
of cascades. Actually the principal importance of this visit and 
other interchanges during the summer of 1941 lay not in accurate 

Progress to December 1941 71 

scientific data but in the general scientific impressions. The 
British, particularly J. Chadwick, were convinced that a U-235 
chain reaction could be achieved. They knew that several kilo- 
grams of heavy water a day were being produced in Norway, 
and that Germany had ordered considerable quantities of par- 
affin to be made using heavy hydrogen; it was difficult to imagine 
a use for these materials other than in work on the uranium 
problem. They feared that if the Germans got atomic bombs 
before the Allies did, the war might be over in a few weeks. The 
sense of urgency which Pegram and Urey brought back with them 
was of great importance. 


4.48. The appointment of a National Academy committee was 
mentioned in Chapter III. The committee's first report in May 
1941 mentioned (a) radioactive poisons, (b) atomic power, and 
(c) atomic bombs, but the emphasis was on power. The second 
report stressed the importance of the new results on plutonium, 
but was not specific about the military uses to which the fission 
process might be put. Both these reports urged that the project 
be pushed more vigorously. 

4.49. The third report (November 6, 1941) was specifically 
concerned with the "possibilities of an explosive fission reaction 
with U-235." Although neither of the first two National Academy 
reports indicated that uranium would be likely to be of decisive 
importance in the present war, this possibility was emphasized 
in the third report. We can do no better than quote portions of 
this report. 

"Since our last report, the progress toward separation of the 
isotopes of uranium has been such as to make urgent a considera- 
tion of (1) the probability of success in the attempt to produce a 
fission bomb, (2) the destructive effect to be expected from such 
a bomb, (3) the anticipated time before its development can be 
completed and production be underway, and (4) a preliminary 
estimate of the costs involved." 

72 Progress to December 1941 

"1. Conditions for a fission bomb. A fission bomb of superlatively 
destructive power will result from bringing quickly together a sufficient 
mass of element U-235. This seems to be as sure as any untried 
prediction based upon theory and experiment can be. Our 
calculations indicate further that the required masses can be 
brought together quickly enough for the reaction to become 
efficient . . . 

"2. Destructive effect of fission bombs, (a) Mass of the bomb. The 
mass of U-235 required to produce explosive fission under appropriate con- 
ditions can hardly be less than 2 kg nor greater than 100 kg. These wide 
limits reflect chiefly the experimental uncertainty in the capture 
cross section of U-235 for fast neutrons . . . (b) Energy released 
by explosive fission. Calculations for the case of masses properly- 
located at the initial instant indicate that between 1 and 5 per 
cent of the fission energy of the uranium should be released at a 
fission explosion. This means from 2 to 10 X 10 8 kilocalories per 
kg of uranium 235. The available explosive energy per kg of uranium 
is thus equivalent to about 300 tons of TNT. 

"3. Time required for development and production of the necessary 
U-235. (a) Amount of uranium needed. Since the destructiveness of 
present bombs is already an important factor in warfare, it is 
evident that, if the destructiveness of the bombs is thus increased 
10,000-fold, they should become of decisive importance. 

"The amount of uranium required will, nevertheless, be large. 
If the estimate is correct that 500,000 tons of TNT bombs would 
be required to devastate Germany's military and industrial 
objectives, from 1 to 10 tons of U-235 will be required to do the same job. 

"(b) Separation of U-235. The separation of the isotopes of uranium 
can be done in the necessary amounts. Several methods are under 
development, at least two of which seem definitely adequate, 
and are approaching the stage of practical test. These are the 
methods of the centrifuge and of diffusion through porous bar- 
riers. Other methods are being investigated or need study which 
may ultimately prove superior, but are now farther from the 
engineering stage. 

"(c) Time required for production of fission bombs. An estimate of 

Progress to December 1941 73 

time required for development, engineering and production of 
fission bombs can be made only very roughly at this time. 

"If all possible effort is spent on the program, one might 
however expect fission bombs to be available in significant 
quantity within three or four years. 

"4. Rough estimate of costs. (The figures given in the Academy 
report under this heading were recognized as only rough esti- 
mates since the scientific and engineering data to make them 
more precise were not available. They showed only that the 
undertaking would be enormously expensive but still in line with 
other war expenditures.)" 

4.50. The report then goes on to consider immediate require- 
ments and desirable reorganization. 


4.51. At the end of Chapter I we summarized the knowledge 
of nuclear fission as of June 1940, and in Chapter II we stated 
the outstanding problems as of the same date. In the light of 
these statements we wish to review the eighteen months' progress 
that has just been recounted. The tangible progress was not great. 
No chain reaction had been achieved; no appreciable amount 
of U-235 had been separated from U-238; only minute amounts of 
Pu-239 had been produced; the production of large quantities of 
uranium metal, heavy water, beryllium, and pure graphite was 
still largely in the discussion stage. But there had been progress. 
Constants were better known; calculations had been checked 
and extended; guesses as to the existence and nuclear properties 
of Pu-239 had been verified. Some study had been made of engi- 
neering problems, process effectiveness, costs, and time schedules. 
Most important of all, the critical size of the bomb had been 
shown to be almost certainly within practical limits. Altogether 
the likelihood that the problems might be solved seemed greater 
in every case than it had in 1 940. Perhaps more important than 
the actual change was the psychological change. Possibly Wigner, 
Szilard, and Fermi were no more thoroughly convinced that 
atomic bombs were possible than the> r had been in 1940, but 

74 Progress to December 1941 

many other people had become familiar with the idea and its 
possible consequences. Apparently, the British and the Germans, 
both grimly at war, thought the problem worth undertaking. 
Furthermore, the whole national psychology had changed. 
Although the attack at Pearl Harbor was yet to come, the im- 
pending threat of war was much more keenly felt than before, 
and expenditures of effort and money that would have seemed 
enormous in 1940 were considered obviously necessary precau- 
tions in December 1941. Thus it was not surprising that Bush and 
his associates felt it was time to push the uranium project vigor- 
ously. For this purpose, there was created an entirely new 
administrative organization which will be described in the next 

HISTORY 1942-1945 

5.1. In Chapter III the administrative history of the uranium 
work up to December 1 941 was reviewed. Chapter IV reported 
the progress of the scientific work up to the same date. The 
present chapter describes the administrative reorganization that 
took place in December 1941 and various changes that occurred 
after that time. 


5.2. Two major decisions were required in the further plan- 
ning of the uranium or atomic-bomb program. These decisions 
were made by Vannevar Bush, Director of the Office of Scientific 
Research and Development (which included NDRC), after 
conference with various scientists and administrators concerned. 
(See Chapter III.) The decisions were: first, that the possibility 
of obtaining atomic bombs for use in the present war was great 
enough to justify an "all out" effort for their development; second, 
that the existing organization, the NDRC Uranium Section 
(known as the S-l Section, and consisting of L. J. Briggs, chair- 
man; G. B. Pegram, vice-chairman; H. T. Wensel, technical 
aide; S. K. Allison, J. W. Beams, G. Breit, E. U. Condon, R. 
Gunn, H. D. Smyth, and H. C. Urey) was not properly organized 
for such an effort. 

5.3. At a meeting of the National Defense Research Committee 
on November 28, 1941, Dr. Bush explained why he felt that it 
was desirable to set up the uranium program outside NDRC. 
The members of NDRC agreed to a transfer. Accordingly, the 
NDRC as an organization had no further connection with the 


76 Administrative History 1942-1945 

uranium program, which was administered for some time 
thereafter by the OSRD directly through an OSRD S-l Section, 
and later through an OSRD S-l Executive Committee. 

5.4. At a meeting of the S-l Section of OSRD on December 6, 
1941, J. B. Conant, speaking for Bush, announced the proposed 
"all out" effort and the reorganization of the group. The S-l 
Section itself had not been formally consulted on the proposed 
reorganization, but there is no doubt that most of its members 
were strongly in favor of the new proposals. The membership of 
the reorganized S-l Section was as follows: J. B. Conant, repre- 
sentative of V. Bush; L. J. Briggs, chairman; G. B. Pegram, vice- 
chairman; A. H. Compton, program chief; E. O. Lawrence, 
program chief; H. C. Urey, program chief; E. V. Murphree, 
chairman of the separately organized Planning Board; H. T. 
Wensel, technical aid; S. K. Allison, J. W. Beams, G. Breit, 
E. U. Condon, H. D. Smyth. 


5.5. At the time the S-l Section was reorganized, Bush also 
set up a Planning Board to be responsible for the technical and 
engineering aspects of the work, for procurement of materials 
and for construction of pilot plants and full-size production 
plants. This Planning Board consisted of E. V. Murphree (chair- 
man), W. K. Lewis, L. W. Chubb, G. O. Curme, Jr., and P. C. 


5.6. It was arranged that contracts for the scientific parts of 
the work would be recommended to Bush not by the full S-l 
Section but by Briggs and Conant after conferences with the 
program chiefs involved and that recommendations on engineer- 
ing contracts would be made to Bush by the Planning Board. 
(The contracts which had been made on behalf of the old 
Uranium Section had been administered through the NDRC.) 

Administrative History 1942-1945 77 

Contracts for the development of diffusion and centrifuge sepa- 
ration processes were to be recommended by the Planning Board, 
which would be responsible for the heavy-water production 
program also. Bush stated that the Planning Board "will be 
responsible for seeing to it that we have plans on which to proceed 
with the next step as expeditiously as possible." 

5.7. The scientific aspects of the work were separated from the 
procurement and engineering phases. The Program Chiefs 
H. C. Urey, E. O. Lawrence, and A. H. Compton were to have 
charge of the scientific aspects. Initially it was proposed that Urey 
should have charge of the separation of isotopes by the diffusion 
and the centrifuge methods and of the research work on the 
production of heavy water. Lawrence was to have charge of the 
initial production of small samples of fissionable elements, of 
quantity production by electromagnetic-separation methods, and 
of certain experimental work relating to the properties of the 
plutonium nucleus. Compton was to have charge of fundamental 
physical studies of the chain reaction and the measurement of 
nuclear properties with especial reference to the explosive chain 
reaction. As an afterthought, he was authorized to explore also 
the possibility that plutonium might be produced in useful 
amounts by the controlled chain-reaction method. It was under- 
stood, however, that this division of responsibility was to be more 
precisely defined in later conferences. (The written records of 
that period do not always give adequate accounts of what was in 
the minds of the men concerned. In deference to security re- 
quirements, references to the importance of plutonium and even 
to the bomb itself were often omitted entirely.) 

5.8. The effect of the reorganization was to put the direction 
of the projects in the hands of a small group consisting of Bush, 
Conant, Briggs, Compton, Urey, Lawrence, and Murphree. 
Theoretically, Compton, Lawrence, Urey, and Murphree were 
responsible only for their respective divisions of the program. 
Each met with Conant and Briggs or occasionally with 
Bush to discuss his specific problems, or even the overall 

78 Administrative History 1942-1945 


5.9. A meeting of the Top Policy Group, consisting of Vice- 
President Henry A. Wallace, Secretary of War Henry L. Stimson, 
and Dr. V. Bush, was held on December 16, 1941 . General George 
C. Marshall and Dr. J. B. Conant, also members of the group, 
were absent; Mr. H. L. Smith of the Budget Bureau attended. 
Bush described the reorganization that was in progress and his 
plans were approved. In a memorandum to Conant describing 
this meeting, Bush wrote, "It was definitely felt by the entire 
group that OSRD should press as fast as possible on the funda- 
mental physics and on the engineering planning, and particularly 
on the construction of pilot plants." Bush estimated the cost of 
this aspect of the work would be four or five million dollars, and 
stated the Army should take over when full-scale construction 
was started, presumably when pilot plants were ready. He sug- 
gested the assignment of a technically trained Army officer to 
become familiar with the general nature of the uranium problem. 
It was made clear at this meeting that the international relations 
involved were in the hands of the President, with Bush responsible 
for liaison on technical matters only. 


5.10. On December 18, 1941, a meeting of the reorganized S-l 
Section was held. Conant was present and discussed the new 
policy, which called for an all-out effort. He emphasized that 
such an effort was justified only by the military value of atomic 
bombs and that all attention must be concentrated in the direc- 
tion of bomb development. The whole meeting was pervaded 
by an atmosphere of enthusiasm and urgency. Several methods 
of electromagnetic separation were proposed and discussed, and 
a number of new contracts were recommended. 


5.11. Another meeting of the OSRD S-l Section was held on 
January 16, 1942. Informal discussions of the various production 

Administrative History 1942-1945 79 

methods took place, and tentative estimates were made as to 
when each method would produce results. These forecasts 
actually were no more than guesses since at that time the scientific 
information available was very incomplete and the problems of 
applying such data as did exist to the construction and operation 
of production plants had hardly been approached. 


5.12. In the middle of January 1942, Compton decided to 
concentrate the work for which he was responsible at the Uni- 
versity of Chicago. The Columbia group under Fermi and its 
accumulated material and equipment and the Princeton group 
which had been studying resonance absorption were moved to 
Chicago in the course of the spring. Certain smaller groups else- 
where remained active under Compton's direction. Under 
Lawrence the investigation of large-scale electromagnetic 
separation was accelerated at the University of California at 
Berkeley and a related separation project was started at Princeton. 
Research and development on the diffusion process and on the 
production of heavy water continued at Columbia under Urey; 
under the general supervision of Murphree, the centrifuge work 
continued at the University of Virginia under Beams while the 
Columbia centrifuge work was transferred to the laboratories of 
the Standard Oil Development Co. at Bayway, New Jersey. 

ON MARCH 9, 1942 

5.13. In a report dated February 20, 1942, Conant recom- 
mended that all phases of the work be pushed at least until July 
1, 1942. Similarly, on March 9, 1942, Dr. Bush sent a report to 
the President reflecting general optimism but placing proper 
emphasis on the tentative nature of conclusions. His report 
contemplated completion of the project in 1 944. In addition, the 
report contained the suggestion that the Army be brought in 
during the summer of 1 942 for construction of full-scale plants. 

80 Administrative History 1942-1945 


5.14. The entire heavy-water program was under review in 
March and April 1942. The reviews followed a visit to the United 
States in February and March 1942 by F. Simon, H. Halban, 
and W. A. Akers from England. In a memorandum of April 1, 
1942 addressed to Bush, Conant reviewed the situation and re- 
ported on conferences with Compton and Briggs. His report 
pointed out that extremely large quantities of heavy water 
would be required for a plutonium production plant employing 
heavy water instead of graphite as a moderator. For this reason, 
he reported adversely on the suggestion that Halban be invited 
to bring to this country the 165 liters of heavy water which he then 
had in England. 

5.15. In a memorandum written to Bush on May 14, 1942 
(shortly before a proposed meeting of Program Chiefs), Conant 
estimated that there were five separation or production methods 
which were about equally likely to succeed: the centrifuge, dif- 
fusion, and electromagnetic methods of separating U-235; the 
uranium-graphite pile and the uranium-heavy-water pile methods 
of producing plutonium. All were considered about ready for 
pilot plant construction and perhaps even for preliminary design 
of production plants. If the methods were to be pushed to the 
production stage, a commitment of five hundred million dollars 
would be entailed. Although it was too early to estimate the 
relative merits of the different methods accurately, it was pre- 
sumed that some methods would prove to be more rapid and 
efficient than others. It was feared, however, that elimination 
of any one method might result in a serious delay. It was thought 
that the Germans might be some distance ahead of the United 
States in a similar program. 

5.16. Conant emphasized a question that has been crucial 
throughout the development of the uranium project. The ques- 
tion was whether atomic bombs would be decisive weapons or 
merely supplementary weapons. If they were decisive, there was 
virtually no limit to the amount of effort and money that should 

Administrative History 1942-1945 81 

be put into the work. But no one knew how effective the atomic 
bombs would be. 


5.17. In May 1942, Conant suggested to Bush that instead of 
encouraging members of the section individually to discuss their 
own phases of the work with Conant and Briggs, the OSRD S-l 
Section should meet for general discussions of the entire program. 
Bush responded by terminating the OSRD S-l Section and 
replacing it with the OSRD S-l Executive Committee, consisting 
of the following: J. B. Conant, chairman, L. J. Briggs, A. H. 
Compton, E. O. Lawrence, E. V. Murphree, H. C. Urey. H. T. 
Wensel and I. Stewart were selected to sit with the Committee 
as technical aide and secretary respectively. 

5.18. The following members of the old OSRD S-l Section 
were appointed as consultants to the new Committee: S. K. 
Allison, J. W. Beams, G. Breit, E. U. Condon, H. D. Smyth. 

5.19. The functions of the new OSRD S-l Executive Com- 
mittee were: (a) To report on the program and budget for the 
next eighteen months, for each method, (b) To prepare recom- 
mendations as to how many programs should be continued. 
(c) To prepare recommendations as to what parts of the program 
should be eliminated. 

5.20. Recommendations relative to matters of OSRD S-l 
policy and relative to the letting of OSRD S-l contracts were 
made on the basis of a majority vote of the Committee. Conant 
refrained from voting except in case of a tie vote. While Bush 
alone had the authority to establish OSRD policies and commit 
OSRD funds, he ordinarily followed the recommendations of 
the S-l Executive Committee. 

ON JUNE 17, 1942 

5.21. On June 13, 1942, Bush and Conant sent to Vice- 
President Henry A. Wallace, Secretary of War Henry L. Stimson, 

82 Administrative History 1942-1945 

and Chief of Staff General George C. Marshall a report recom- 
mending detailed plans for the expansion and continuation of 
the atomic-bomb program. All three approved the report. On 
June 17, 1942, the report was sent by Bush to the President, who 
also approved. The report, w r hich is too long to present in full, 
contained four principal parts, which dealt with: (a) The status 
of the development as appraised by the senior scientists; (b) 
Recommendations by the program chiefs and Planning Board; 
(c) Comments by Bush, Conant, and Maj. Gen. W. D. Styer; (d) 
Recommendations by Bush and Conant. We may paraphrase 
parts (a) and (c) as follows: 

(a) The status of the program. (1) It was clear that an amount of 
U-235 or plutonium comprising a number of kilograms would be 
explosive, that such an explosion would be equivalent to several 
thousand tons of TNT, and that such an explosion could be 
caused to occur at the desired instant. (2) It was clear that there 
were four methods of preparing the fissionable material and that 
all of these methods appeared feasible; but it was not possible to 
state definitely that any given one of these is superior to the others. 
(3) It was clear that production plants of considerable size could 
be designed and built. (4) It seemed likely that, granted adequate 
funds and priorities, full-scale plant operation could be started 
soon enough to be of military significance. 

(c) Comments by Bush, Conant , and General Styer. Certain recom- 
mendations had been made by Lawrence, Urey, Compton, and 
Murphree. These recommendations had been reviewed by Bush, 
Conant, and General Styer (who was instructed by General 
Marshall to follow the progress of the program) and their com- 
ments concerning the program were as follows: (1) If four sepa- 
rate methods all appeared to a highly competent scientific group 
to be capable of successful application, it appeared certain that 
the desired end result could be attained by the enemy, provided 
he had sufficient time. (2) The program as proposed obviously 
could not be carried out rapidly without interfering with other 
important matters, as regards both scientific personnel and 
critical materials. A choice had to be made between the military 

Administrative History 1942-1945 83 

result which appeared attainable and the certain interference 
with other war activities. (3) It was unsafe at that time, in view 
of the pioneering nature of the entire effort, to concentrate on 
only one means of obtaining the result. (4) It therefore appeared 
best to proceed at once with those phases of the program which 
interfered least with other important war activities. Work on 
other phases of the program could proceed after questions of 
interference were resolved. 

5.22. The June 13, 1942, report to the President and Bush's 
transmittal letter dated June 17, 1942, were returned to Bush 
with the initialled approval of the President. A copy of the report 
was then sent by Bush to General Styer on June 19, 1942. 


5.23. On June 18, 1942, Colonel J. C. Marshall, Corps of 
Engineers, was instructed by the Chief of Engineers to form a 
new district in the Corps of Engineers to carry on special work 
(atomic bombs) assigned to it. This district was designated the 
Manhattan District and was officially established on August 13, 
1942. The work with which it was concerned was labeled, for 
security reasons, the "DSM Project" (Development of Substitute 


5.24. On September 17, 1942, the Secretary of War placed 
Brigadier General L. R. Groves of the Corps of Engineers in 
complete charge of all Army activities relating to the DSM 


5.25. A conference was held on September 23, 1942, among 
those persons designated by the President to determine the 
general policies of the project, and certain others. Those present 
were Secretary of War Henry L. Stimson, Chief of Staff General 

84 Administrative History 1942-1945 

George C. Marshall, Dr. J. B. Conant, Dr. V. Bush, Major 
General Brehon Somervell, Major General W. D. Styer, and 
Brigadier General L. R. Groves. (Vice-President Henry A. 
Wallace was unable to attend.) A Military Policy Committee 
was appointed consisting of Dr. V. Bush as Chairman with Dr. 
J. B. Conant as his alternate, Major General W. D. Styer, and 
Rear Admiral W. R. Purnell. General Groves was named to sit 
with the committee and act as Executive Officer to carry out the 
policies that were determined. The duties of this committee were 
to plan military policies relating to materials, research and 
development, production, strategy, and tactics, and to submit 
progress reports to the policy group designated by the President. 

5.26. The appointment of the Military Policy Committee was 
approved by the Joint New Weapons Committee, established by 
the U. S. Joint Chiefs of Staff and consisting of Dr. V. Bush, Rear 
Admiral W. R. Purnell, and Brigadier General R. G. Moses. 

5.27. The creation of the Military Policy Committee in effect 
placed all phases of the DSM Project under the control of Dr. 
Bush, Dr. Conant, General Styer, Admiral Purnell, and General 

5.28. The OSRD S-l Executive Committee held meetings 
about once every month from June 1 942 to May 1 943 and once 
after that time, in September 1943. These meetings were normally 
attended by General Groves, after September 1942, and Colonel 
Marshall, and frequently by representatives of the industrial 
companies concerned with the production plants. Recommenda- 
tions of the Committee were not binding but were usually 
followed. Thus it served as an advisory body to Dr. Bush and 
General Groves, and as an initial liaison group between the 
scientific, industrial, and military parts of the DMS Project. 
The S-l Executive Committee has never been formally dissolved, 
but it has been inactive since the fall of 1943. 

5.29. The procurement and engineering functions of the 
Planning Board were taken over by the Manhattan District in 
the summer of 1942 and that board then became inactive. 

5.30. By the spring of 1943 it was felt that the Manhattan 

Administrative History 1942-1945 85 

District was in a position to take over research and development 
contracts from the OSRD. Such a transfer was effected as of 
May 1, 1943, and marked the end of the formal connection of 
OSRD with the uranium project. 

5.31. In July 1943 Conant and R. G. Tolman were formally 
asked by General Groves to serve as his scientific advisers. They 
had already been doing so informally and have continued to do 
so. Coordination of the various scientific and technical programs 
was accomplished by meetings between General Groves and the 
leaders of the various projects, in particular, Compton, Lawrence, 
Oppenheimer (see Ghapter XII), and Urey. 


5.32. Since 1943 there have been no important changes in 
the form of the organization and few of importance in the operat- 
ing personnel. General Groves has continued to carry the major 
responsibility for correlating the whole effort and keeping it 
directed toward its military objectives. It has been his duty to 
keep the various parts of the project in step, to see that raw 
materials were available for the various plants, to determine 
production schedules, to make sure that the development of 
bomb design kept up with production schedules, to arrange for 
use of the bombs when the time came, and to maintain an ade- 
quate system of security. In discharging these duties General 
Groves has had the help of his tremendous organization made up 
of civilian scientists and engineers and Engineer officers and 
enlisted men. Many of the civilians have been mentioned already 
or will be mentioned in later chapters dealing with particular 
projects. Brigadier General T. F. Farrell has acted as General 
Groves' deputy in the important later phases of the project. 
Colonel K. D. Nichols, the District Engineer of the Manhattan 
District with his headquarters at the Clinton Engineer Works, has 
been connected with the project since 1942. He has been con- 
cerned with the research and production problems of both U-235 

86 Administrative History 1942-1945 

and plutonium and has always shown exceptional understanding 
of the technical problems and their relative importance. Two 
other officers who should be mentioned are Colonel F. T. Mat- 
thias and Colonel S. L. Warren. Colonel Matthias has discharged 
major responsibilities at the Hanford Engineer Works in an 
extremely able manner; his duties have been concerned with 
both the construction and operational phases of the project. 
Colonel Warren is chief of the Medical Section of the Manhattan 
District and therefore has had ultimate responsibility for health 
problems in all parts of the project. 


5.33. By the end of 1941 an extensive review of the whole 
uranium situation had been completed. As a result of this review 
Bush and his advisers decided to increase the effort on the 
uranium project and to change the organization. This decision 
was approved by President Roosevelt. From January 1 942 until 
early summer of 1942 the uranium work was directed by Bush 
and Conant working with the Program Chiefs and a Planning 
Board. In the summer of 1942 the Army, through the Corps of 
Engineers, was assigned an active part in the procurement and 
engineering phases, organizing the Manhattan District for the 
purpose. In September 1942, Dr. Bush, Dr. Conant, General 
Styer, and Admiral Purnell were appointed as a Military Policy 
Committee to determine the general policies of the whole project. 
Also in September, General Groves was appointed to take charge 
of all Army activities of the project. The period of joint OSRD 
and Army control continued through April 1943 with the Army 
playing an increasingly important role as the industrial effort got 
fully under way. In May 1943 the research contracts were trans- 
ferred to the Corps of Engineers; the period of joint OSRD- Army 
control ended and the period of complete Army control began. 

5.34. Since the earliest days of the project, President Roosevelt 
had followed it with interest and, until his death, continued 
to study and approve the broad programs of the Military Policy 

Administrative History 1942-1945 87 

Committee. President Truman, who as a United States Senator 
had been aware of the project and its magnitude, was given the 
complete up-to-date picture by the Secretary of War and General 
Groves at a White House conference immediately after his 
inauguration. Thereafter the President gave the program his 
complete support, keeping in constant touch with the progress. 



6.1. As has been made clear in Chapters IV and V, the infor- 
mation accumulated by the end of 1941 as to the possibility of 
producing an atomic bomb was such as to warrant expansion of 
the w r ork, and this expansion called for an administrative reor- 
ganization. It was generally accepted that there was a very high 
probability that an atomic bomb of enormous destructive power 
could be made, either from concentrated U-235 or from the new 
element plutonium. It was proposed, therefore, to institute an 
intensive experimental and theoretical program including work 
both on isotope separation and on the chain-reaction problems. 
It was hoped that this program would establish definitely whether 
or not U-235 could be separated in significant quantities from 
U-238, either by electromagnetic or statistical methods; whether 
or not a chain reaction could be established with natural uranium 
or its compounds and could be made to yield relatively large 
quantities of plutonium; and whether or not the plutonium so 
produced could be separated from the parent material, uranium. 
It was hoped also that the program would provide the theo- 
retical and experimental data required for the design of a fast- 
neutron chain-reacting bomb. 

6.2. As has been explained in Chapter V, the problems of 
isotope separation had been assigned to groups under Lawrence 
and Urey while the remaining problems were assigned to Comp- 
ton's group, which was organized under the cryptically named 
"Metallurgical Laboratory" of the University of Chicago. In 
this chapter and the following two chapters we shall describe the 
work of the Metallurgical Laboratory and the associated labora- 
tories up to June 1945. In later chapters we shall discuss isotope- 


Metallurgical Project in 1942 89 

separation work and the work of the bomb development group, 
which was separated from the Metallurgical Laboratory early 
in 1943. 

6.3. It would be futile to attempt an assessment of the relative 
importance of the contributions of the various laboratories to the 
overall success of the atomic-bomb project. This report makes no 
such attempt, and there is little correlation between the space 
devoted to the work of a given group and the ability or importance 
of that group. In deciding which subdivision of the atomic-bomb 
project should be discussed first and most fully, we have been 
governed by criteria of general interest and of military security. 
Some developments of great technical importance are of little 
ueneral interest; others both interesting and important must still 
be kept secret. Such criteria, applied to the objectives and ac- 
complishments of the various laboratories set up since large-scale 
work began, favor the Metallurgical Laboratory as the part of 
the project to be treated most completely. 


6.4. In accordance with the general objectives just outlined, 
the initial objectives of the Metallurgical Laboratory were: first, 
to find a system using normal uranium in which a chain reaction 
would occur; second, to show that, if such a chain reaction did 
occur, it would be possible to separate plutonium chemically from 
the other material; and, finally, to obtain the theoretical and 
experimental data for effecting an explosive chain reaction with 
either U-235 or with plutonium. The ultimate objective of the 
laboratory was to prepare plans for the large-scale production of 
plutonium and for its use in bombs. 


6.5. The laboratory had not only to concern itself with its 
immediate objectives but simultaneously to bear in mind the 
ultimate objectives and to work toward them on the assumption 
that the immediate objectives would be attained. It could not 
wait for a chain reaction to be achieved before studying the 

90 Metallurgical Project in 1942 

chemistry of plutonium. It had to assume that plutonium would 
be separated and to go ahead with the formulation of plans for 
its production and use. Consequently problems were continually 
redefined as new information became available, and research 
programs were reassessed almost from week to week. In a general 
way the experimental nuclear physics group under E. Fermi was 
primarily concerned with getting a chain reaction going, the 
chemistry division organized by F. H. Spedding (later in turn 
under S. K. Allison, J. Franck, W. C. Johnson, and T. Hogness) 
with the chemistry of plutonium and with separation methods, 
and the theoretical group under E. Wigner with designing pro- 
duction piles. However, the problems were intertwined and the 
various scientific and technical aspects of the fission process were 
studied in whatever group seemed best equipped for the particu- 
lar task. In March 1942, Thomas Moore was brought in to 
head the engineering group. Other senior men in this group were 
M. C. Leverett, J. A. Wheeler and C. M. Cooper, who later 
succeeded Moore as head of the Technical Division. In the 
summer of 1942 the importance of health problems became 
apparent and a Health Division was organized under Dr. R. S. 
Stone. The difficult task of organizing and administering a re- 
search laboratory growing in size and complexity with almost 
explosive violence was carried out by R. L. Doan as Laboratory 

6.6. We have chosen to confine this chapter to the work of 
1942 because a self-sustaining chain reaction was first achieved 
on December 2 of that year, at a time when the whole Chicago 
project was being appraised by a reviewing committee with the 
members particularly selected for their engineering background. * 
That was a dramatic coincidence and also a convenient one for 
purposes of this report since either incident might be considered 
to mark the end of an epoch at the Metallurgical Laboratory. 
Furthermore, in preparation for the reviewing committee's visit 

* This committee was composed of W. K. Lewis, C. H. Greenewalt, 
T. C. Gary, and Roger Williams. E. V. Murphree was also a member but 
due to illness was unable to participate. 

Metallurgical Project in 1942 91 

a comprehensive report had been prepared. That report was 
generally known as the "Feasibility Report" and has been used 
extensively in preparing this chapter. 


6.7. In this chapter we shall present the material in the order 
of the objectives given above. In Part I we shall discuss progress 
towards the initial objectives, including (a) procurement of 
materials, (b) the experimental proof of the chain reaction, (c) the 
chemistry of plutonium and some of the problems of separation, 
(d) some of the types of auxiliary experiments that were per- 
formed, and finally (e) the "fast neutron" work. Necessarily 
the work described in detail is only a sampling of the large amount 
of theoretical and experimental work actually performed. In 
Part II we shall discuss the possibilities that were considered for 
production piles and separation methods, and the specific pro- 
posals made in November 1942. 




6.8. It has been made clear in earlier chapters of this report 
that the procurement of materials of sufficient purity was a major 
part of the problem. As far as uranium was concerned, it seemed 
likely that it would be needed in highly purified metallic form 
or at least as highly purified uranium oxide. The other materials 
which were going to be needed were either graphite, heavy water, 
or possibly beryllium. It was clear at this time that, however 
advantageous heavy water might be as a moderator, no large 
quantities of it would be available for months or years. Beryllium 
seemed less advantageous and almost as difficult to get. There- 
fore the procurement efforts for a moderator were centered on 
graphite. As has been explained in Chapter V, procurement of 

92 Metallurgical Project in 1942 

uranium and graphite was not primarily the responsibility of 
the Metallurgical Laboratory but was handled through E. V. 
Murphree and others on the "planning board." In fact, the 
obvious interest of the Metallurgical Laboratory in the problem 
led to continual intervention by its representatives. A great deal 
of the credit for the eventual success in obtaining materials is due 
to N. Hilberry and later R. L. Doan, always supported by 
A. H. Compton. 


6.9. Obviously there would be no point in undertaking this 
whole project if it were not going to be possible to find enough 
uranium for producing the bombs. Early indications were favor- 
able, and a careful survey made in November 1942 showed that 
immediate delivery could be made of adequate tonnages of 
uranium ores. 


6.10. At the end of 1941 the only uranium metal in existence 
was a few grams of good material made on an experimental basis 
by the Westinghouse Electric and Manufacturing Company and 
others and a few pounds of highly impure pyrophoric powder 
made by Metal Hydrides Company. The only considerable 
amount of raw material then available in this country was in the 
form of a commercial grade of black uranium oxide, which could 
be obtained in limited quantities from the Canadian Radium 
and Uranium Co. It contained 2 to 5 per cent of impurities and 
was the material which gave a neutron multiplication factor of 
only about 0.87 when used in an exponential pile. 

6.11. By May 1942, deliveries averaging 15 tons a month of 
black oxide of higher purity and more uniform grade started 
coming in. Total impurities were less than 1 per cent, boron 
comprised a few parts per million, and the neutron multiplication 
factor (k) was about 0.98. (It is to be remembered that the 
multiplication factor depends also on the purity of the graphite.) 
Deliveries of this material reached a ton a day in September 1942. 

Metallurgical Project in 1942 93 

6.12. Experiments at the National Bureau of Standards by 
J. I. Hoffman demonstrated that, by the use of an ether extraction 
method, all the impurities are removed by a single extraction of 
uranyl nitrate. The use of this method removed the great bulk 
of the difficulties in securing pure oxide and pure materials for 
the production of metal. Early in May 1942, arrangements were 
completed with the Mallinckrodt Chemical Works in St. Louis 
to put the new grade of oxide through an ether extraction process 
on a production basis for a further reduction in impurity content 
and to deliver the final product as brown dioxide. Deliveries 
started in July 1942 at a rate of 30 tons a month. This oxide is 
now used as a starting point for all metal production, and no 
higher degree of purity can be expected on a commercial scale. 
In fact, it was a remarkable achievement to have developed and 
put into production on a scale of the order of one ton per day a 
process for transforming grossly impure commercial oxide to 
oxide of a degree of purity seldom achieved even on a laboratory 

6.13. The process which Westinghouse had been using to pro- 
duce the metal was the electrolysis of KUFs at a cost of about 
$1,000 a pound. Since the KUFs was produced photochemically 
under the action of sunlight this method constituted a potential 
bottleneck in production. It was found that uranium tetrafluoride 
could be used instead of KUFs, and steps were taken to have this 
salt produced at the Harshaw Chemical Company in Cleveland 
and at the du Pont plant in Penns Grove, New Jersey. Production 
started in August 1942 and by October 1942 was up to 700 
pounds per day at Harshaw and 300 pounds per day at du Pont, 
the method of manufacture in both cases being the hydrofluorina- 
tion of Mallinckrodt-purified dioxide. 

6.14. As the result of this supply of raw materials to Westing- 
house, and as a result of plant expansion, deliveries from Westing- 
house had accumulated to a total of more than 6,000 pounds by 
November 1 942 and were expected to be at the rate of 500 pounds 
per day by January 1943. The purity of the metal was good, and 
the cost had dropped to $22 per pound. 

94 Metallurgical Project in 1942 

6.15. Deliveries of acceptable metal from Metal Hydrides Co. 
were delayed for various reasons and were just beginning in 
November 1942. This company's production was supposed to 
reach a thousand pounds per week thereafter. 

6.16. Neither the Westinghouse process nor the Metal Hy- 
drides Process was entirely satisfactory. Intensive activity designed 
to accelerate metal production, and carried out independently 
by F. H. Spedding and his associates at Iowa State College at 
Ames, Iowa, and by C. J. Rodden at the National Bureau of 
Standards, resulted in the development of a satisfactory method. 
Production facilities were set up at Ames in the fall of 1942 and 
had already produced more than one ton by the end of November. 
The process was extremely simple, rapid and low cost. 

6.17. Further research indicated additional changes that could 
be made to advantage, and by the middle of 1943 Spedding at 
Iowa and other producers who entered the picture were using 
the final production method adopted. 

6.18. By the end of 1942 arrangements had been made by the 
Manhattan District to increase metal production by making 
greater use of the Mallinckrodt Chemical Works, the Union 
Carbide and Carbon Corporation, and the du Pont Company. 

6.19. To summarize, almost no metal was available during 
most of 1942, a fact that seriously delayed progress as we shall 
see, but the production problems had been nearly solved by the 
end of 1 942 and some 6 tons of metal were incorporated in the 
pile built in November 1942. The whole problem of procurement 
of metal was taken over by the Manhattan District at the end 
of the year, under the general direction of Colonel RuhofT, 
formerly with the Mallinckrodt Chemical Works. From the point 
of view of the Metallurgical Project no further serious delays or 
difficulty have occurred because of metal shortages. 


6.20. At the beginning of 1942 graphite production was still 
unsatisfactory but it was, of course, in quite a different condition 
from the metal production since the industrial production of 

Metallurgical Project in 1942 95 

graphite had already been very large. The problem was merely 
one of purity and priority. Largely through the efforts of N. Hil- 
berry, the National Carbon Company and the Speer Carbon 
Company were both drawn into the picture. Following sugges- 
tions made by the experts of the National Bureau of Standards, 
these companies were able to produce highly purified graphite 
with a neutron absorption some 20 per cent less than the standard 
commercial materials previously used. Although efforts further to 
reduce the impurities have had some success, the purity problem 
was essentially solved by the middle of 1942 and large orders 
were placed with the cooperation of the War Production Board. 
As in the case of the metal, the graphite procurement problem 
was taken over by the Manhattan District. 



6.21. At the time that the Metallurgical Project was organized, 
most of the physicists familiar with the problem believed that a 
chain-reacting pile probably could be built if sufficiently pure 
graphite and pure uranium metal could be obtained. Enough 
work had been done on resonance absorption, on the theory of 
absorption and diffusion of neutrons hi a pile, and on inter- 
mediate experiments to make it possible to design a lattice struc- 
ture that had a very good chance of maintaining a chain reaction. 
Nevertheless, there were uncertainties in the experimental data 
and in the approximations that had to be made in the theoretical 
calculations. There were two alternatives: (1) to build a pile 
according to the best possible design; (2) to make more accurate 
determinations of the pertinent nuclear constants, to perform 
intermediate experiments, and to improve the calculations. There 
is little doubt that the first alternative was the one likely to lead 
most rapidly to the production of plutonium. There were many 
important questions which could have been answered more 
rapidly by such an operating pile than by a series of small-scale 
experiments. Unfortunately, the necessary amounts of materials 

96 Metallurgical Project in 1Q42 

were not available and did not become available for nearly nine 
months. Consequently, it was necessary to choose the second 
alternative, that is, to accumulate all relevant or possibly relevant 
information by whatever means were available. 

6.22. The major line of investigation was a series of inter- 
mediate experiments. The particular set-up for each intermediate 
experiment could be used to test calculations based on separate 
auxiliary experiments. For example, the proportion of uranium 
oxide to graphite was varied, oxides of different purities were 
used, oxide was used in lumps of various sizes and shapes and 
degrees of compression, the lattice spacing was varied, the effect 
of surrounding the uranium oxide units with beryllium and with 
paraffin was tried; and, finally, piles of identical lattice type but 
of different total size were tried to see whether the values of the 
multiplication factor k (for infinite size) calculated from the 
different sets of results were identical. In general, E. Fermi had 
direct charge of investigations of effects of impurities, and S. K. 
Allison had charge of tests involving different lattice dimensions. 
All these experiments strengthened the confidence of the group 
in the calculated value of k and in the belief that a pile could be 
built with k greater than unity. In July enough purified uranium 
oxide from Mallinckrodt was available to permit building inter- 
mediate pile No. 9. As in previous experiments, a radium- 
beryllium neutron source was placed at the bottom of the lattice 
structure and the neutron density measured along the vertical 
axis of the pile. By this time it was known that the neutron density 
decreased exponentially with increasing distance from the neutron 
source (hence the name often used for experiments of this type, 
"exponential pile") and that, from such rates of decrease, the 
multiplication constant k for an infinitely large pile of the same 
lattice proportions could be calculated. For the first time the 
multiplication constant k so calculated from experimental results 
came out greater than one. (The actual value was 1.007.) Even 
before this experiment Compton predicted in his report of July 1 
that a k value somewhere between 1.04 and 1.05 could be ob- 
tained in a pile containing highly purified uranium oxide and 

Metallurgical Project in 1942 97 

graphite, provided that the air was removed from the pile to 
avoid neutron absorption by nitrogen. 


6.23. We shall not mention a majority of the various auxiliary 
experiments done during this period. There was one, however, 
the study of delayed neutrons that we shall discuss because it is 
a good example of the kind of experiment that had to be per- 
formed and because it concerned one effect, not heretofore men- 
tioned, that is of great importance in controlling a chain-reacting 

6.24. From previous investigations, some of which were already 
published, it was known that about 1 per cent of the neutrons 
emitted in fission processes were not ejected immediately but 
were given off in decreasing quantity over a period of time, a 
fact reminiscent of the emission of beta rays from shortlived 
radioactive substances. Several half-lives had been observed, the 
longest being of the order of a minute. 

6.25. It was realized early that this time delay gave a sort of 
inertia to the chain reaction that should greatly facilitate control. 
If the effective multiplication factor of a pile became slightly 
greater than 1, the neutron density would not rise to harmfully 
large values almost instantly but would rise gradually so that 
there would be a chance for controls to operate. (Other time 
intervals involved, such as those between collisions, are too small 
to be useful.) 

6.26. Because of the importance of this effect of delayed neu- 
trons for control it was decided to repeat and improve the earlier 
measurements. (The fact that this was a repetition rather than a 
new measurement is also typical of much of the work in physics 
at this period.) A description of the experiment is given in Ap- 
pendix 3. The results indicated that 1.0 per cent of the neutrons 
emitted in uranium fission are delayed by at least 0.01 second 
and that about 0.7 per cent are delayed by as much as a minute. 
By designing a pile such that the effective value of k, the multi- 

98 Metallurgical Project in 1942 

plication factor, is only 1.01 the number of delayed neutrons is 
sufficient to allow easy control. 


6.27. By the fall of 1942 enough graphite, uranium oxide, and 
uranium metal were available at Chicago to justify an attempt 
to build an actual self-sustaining chain-reacting pile. But the 
amount of metal available was small only about 6 tons and 
other materials were none too plentiful and of varying quality. 
These conditions rather than optimum efficiency controlled the 

6.28. The pile was constructed on the lattice principle with 
graphite as a moderator and lumps of metal or oxide as the 
reacting units regularly spaced through the graphite to form the 
lattice. Instruments situated at various points in the pile or near 
it indicated the neutron intensity, and movable strips of absorbing 
material served as controls. (For a more complete description of 
the pile, see Appendix 4.) Since there were bound to be some 
neutrons present from spontaneous fission or other sources, it 
was anticipated that the reaction would start as soon as the struc- 
ture had reached critical size if the control strips were not set 
in "retard" position. Consequently, the control strips were placed 
in a suitable "retard" position from the start and the neutron 
intensity was measured frequently. This was fortunate since the 
approach to critical condition was found to occur at an earlier 
stage of assembly than had been anticipated. 

6.29. The pile was first operated as a self-sustaining system on 
December 2, 1942. So far as we know, this was the first time that 
human beings ever initiated a self-maintaining nuclear chain 
reaction. Initially the pile was operated at a power level of 
J^ watt, but on December 12 the power level was raised to 200 


6.30. In these experiments no direct measurements of energy 
release were made. The number of neutrons per second emitted 

Metallurgical Project in 1942 99 

by the pile was estimated in terms of the activity of standardized 
indium foils. Then, from a knowledge of the number of neutrons 
produced per fission, the resultant rate of energy release (wattage) 
was calculated. 


6.31. Evidently this experiment, performed on December 2 
just as a reviewing committee was appraising the Chicago project, 
answered beyond all shadow of doubt the first question before the 
Metallurgical Laboratory; a self-sustaining nuclear chain reac- 
tion had been produced in a system using normal uranium. This 
experiment had been performed under the general direction of 
E. Fermi, assisted principally by the groups headed by W. H. Zinn 
and H. L. Anderson. V. C. Wilson and his group had been largely 
responsible for developing the instruments and controls, and a 
great many others in the laboratory had contributed to the 
success of the enterprise. 


6.32. The immediate object of building a uranium-graphite 
pile was to prove that there were conditions under which a chain 
reaction would occur, but the ultimate objective of the laboratory 
was to produce plutonium by a chain reaction. Therefore we are 
interested in the relation between the power at which a pile 
operates and the rate at which it produces plutonium. The rela- 
tion may be evaluated to a first approximation rather easily. 
A pile running stably must be producing as many neutrons as it 
is losing. For every thermal neutron absorbed in U-235 a certain 
number of neutrons, r;, is emitted. One of these neutrons is re- 
quired to maintain the chain. Therefore, assuming the extra 
neutrons all are absorbed by U-238 to form plutonium, there will 
be TJ 1 atoms of Pu 239 formed for every fission. Every fission 
releases roughly 200 Mev of energy. Therefore the formation 
of 77 1 atoms of plutonium accompanies the release of about 
200 Mev. Since 77 1 is a small number, we can guess that to 

100 Metallurgical Project in 1942 

produce a kilogram a day of plutonium a chain-reacting pile 
must be releasing energy at the rate of 500,000 to 1,500,000 kilo- 
watts. The first chain-reacting pile that we have just described 
operated at a maximum of 200 watts. Assuming that a single 
bomb will require the order of one to 100 kilograms of plutonium, 
the pile that has been described would have to be kept going at 
least 70,000 years to produce a single bomb. Evidently the prob- 
lem of quantity production of plutonium was not yet solved. 


6.33. The second specific objective of the Metallurgical 
Laboratory was to show that, if a chain reaction did occur, it 
would be feasible to separate the plutonium chemically from the 
other material with which it is found. Progress toward this objec- 
tive was necessarily slower than toward the attainment of a chain 
reaction. Initially little was done at the Metallurgical Laboratory 
on chemical problems although the extraction problem was dis- 
cussed in a conference soon after the project was organized and 
the work of Seaborg's group at the University of California on 
plutonium was encouraged. On April 22-23, 1942, a general 
conference on chemistry was held at Chicago, attended by F. H. 
Spedding, E. W. Thiele, G. T. Seaborg, J. W. Kennedy, H. C. 
Urey, E. Wigner, N. Hilberry, G. E. Boyd, I. B.Johns, H. A. Wil- 
helm, I. Perlman, A. C. Wahl, and J. A. Wheeler. Spedding, in 
opening the meeting, pointed out that there were two main tasks 
for the chemists: first, to separate plutonium in the amounts and 
purity required for war purposes; second, to obtain a good under- 
standing of the chemistry necessary for the construction and 
maintenance of the pile. The separation problem was to be studied 
by a new group at Chicago under the direction of Seaborg, by 
Johns and Wilhelm at Ames, and by Wahl and Kennedy con- 
tinuing the work at California. Other closely related groups at 
Chicago were to be C. D. Coryell's, working on the fission prod- 
ucts, and Boyd's on analytical problems. The chemistry group at 
Chicago has grown speedily since that time. A new building had 
to be constructed to house it late in 1942, and this building was 

Metallurgical Project in 1942 101 

enlarged subsequently. Altogether, the solving of many of the 
chemical problems has been one of the most remarkable achieve- 
ments of the Metallurgical Laboratory. 

6.34. The first isotope of plutonium discovered and studied 
was not the 239 isotope but the 238 isotope, which is an alpha-ray 
emitter with a half-life of about 50 years. U-238 bombarded with 
deuterons gives gsNp 238 which disintegrates to 94Pu 238 by beta 
emission.* The first evidence of the actual existence of these new 
elements (ruling out the original erroneous interpretation of the 
splitting of uranium as evidence for their existence) was obtained 
by E. McMillan and P. H. Abelson who isolated 93-238 from 
uranium bombarded with deuterons in the Berkeley cyclotron. 
This new element w r as identified as a beta emitter but the sample 
was too small for isolation of the daughter product 94-238. Later, 
enough Pu-238 was prepared to permit Seaborg, Kennedy and 
\\ ahl to begin the study of its chemical properties in the winter 
of 1940-1941 by using tracer chemistry with carriers according 
to practice usual in radiochemistry. By such studies many 
chemical properties of plutonium were determined, and several 
possible chemical processes were evolved by which Pu-239 might 
be removed from the chain-reacting pile. The success of experi- 
ments on a tracer scale led to plans to produce enough Pu-239 
to be treated as an ordinary substance on the ultra-microchemical 
M-ale. Such quantities were produced by prolonged bombard- 
ment of several hundred pounds of uranyl nitrate with neutrons 
obtained with the aid of cyclotrons, first at Berkeley and later at 
Washington University in St. Louis. By the end of 1942, some- 
tiling over 500 micrograms had been obtained in the form of pure 
plutonium salts. Although this amount is less than would be 
needed to make the head of a pin, for the micro-chemists it was 
sufficient to yield considerable information; for one microgram 
is considered sufficient to carry out weighing experiments, 
dtrations, solubility studies, etc. 

6.35. From its position in the periodic table, plutonium might 
l;>e expected to be similar to the rare earths or to uranium* 

* See drawing on p. 8. 

102 Metallurgical Project in 1942 

thorium, or osmium. Which of these it will resemble most closely 
depends, of course, on the arrangement of the outermost groups 
of electrons and this arrangement could hardly have been pre- 
dicted. On the whole, plutonium turned out to be more like 
uranium than like any of the other elements named and might 
even be regarded as the second member of a new rare-earth 
series beginning with uranium. It was discovered fairly early 
that there were at least two states of oxidation of plutonium. 
(It is now known that there are four, corresponding to positive 
valences of 3, 4, 5, and 6.) Successful microchemical preparation 
of some plutonium salts and a study of their properties led to the 
general conclusion that it was possible to separate plutonium 
chemically from the other materials in the pile. This conclusion 
represents the attainment of the second immediate objective of 
the Metallurgical Laboratory. Thus, by the end of 1942, 
plutonium, entirely unknown eighteen months earlier, was con- 
sidered an element whose chemical behavior was as well under- 
stood as that of several of the elements of the old periodic table. 


6.36. Besides the major problems we have mentioned, i.e., the 
chain reaction, the chemical separation, and the planning for a 
production plant, there were innumerable minor problems to be 
solved. Among the more important of these were the improve- 
ment of neutron counters, ionization chambers, and other instru- 
ments, the study of corrosion of uranium and aluminum by 
water and other possible coolants, the determination of the 
effects of temperature variation on neutron cross sections, the 
fabrication of uranium rods and tubes, the study of fission prod- 
ucts, and the determination of the biological effects of radiation. 
As typical of this kind of work we can cite the development of 
methods of fabricating and coating uranium metal, under the 
direction of E. Creutz. Without the accomplishment of these 
secondary investigations the project could not have reached its 
goal. To give some further idea of the scope of the work, a list 

Metallurgical Project in 1942 103 

of twenty report titles is presented in Appendix 5, the 20 reports 
being selected from the 400 or so issued during 1942. 


6.37. The third initial objective of the Metallurgical Project 
was to obtain theoretical and experimental data on a "fast 
neutron" reaction, such as would be required in an atomic 
bomb. This aspect of the work was initially planned and coordi- 
nated by G. Breit of the University of Wisconsin and later con- 
tinued by J. R. Oppenheimer of the University of California. 
Since the actual construction of the bomb was to be the final part 
of the program, the urgency of studying such reactions was not 
so great. Consequently, little attention was given to the theo- 
retical problems until the summer of 1942, when a group was 
organized at Chicago under the leadership of Oppenheimer. 

6.38. In the meantime experimental work initiated in most 
instances by G. Breit, had been in progress (under the general 
direction of the Metallurgical Project) at various institutions 
having equipment suitable for fast-neutron studies (Carnegie 
Institution of Washington, the National Bureau of Standards, 
Cornell University, Purdue University, University of Chicago, 
University of Minnesota, University of Wisconsin, University of 
California, Stanford University, University of Indiana, and Rice 
Institute). The problems under investigation involved scattering, 
absorption and fission cross section, the energy spectrum of 
fission neutrons, and the time delay in the emission of fission 
neutrons. For the most part this work represented an intermediate 
step in confirming and extending previous measurements but 
reached no new final conclusion. This type of work was subse- 
quently concentrated at another site (see Chapter XII). 

6.39. As indicated by the "Feasibility Report" (in a section 
written by J. H. Manley, J. R. Oppenheimer, R. Serber, and 
E. Teller) the picture had changed significantly in only one 
respect since the appearance of the National Academy Report 
a year earlier. Theoretical studies now showed that the effective- 
ness of the atomic bomb in producing damage would be greater 

104 Metallurgical Project in 1942 

than had been indicated in the National Academy report. How- 
ever, critical size of the bomb was still unknown. Methods of 
detonating the bomb had been investigated somewhat, but on 
the whole no certain answers had been reached. 




6.40. As we have seen, the initial objectives of the Metallurgical 
Laboratory had been reached by the end of 1942, but the ulti- 
mate objectives, the production of large quantities of plutonium 
and the design and fabrication of bombs, were still far from at- 
tained. The responsibility for the design and fabrication of bombs 
was transferred to another group at about this time; its work is 
reported in Chapter XII. The production of Pu-239 in quantity 
has remained the principal responsibility of the Metallurgical 
Laboratory although shared with the du Pont Company since 
the end of 1942. 

6.41 . On the basis of the evidence available it was clear that 
a plutonium production rate somewhere between a kilogram a 
month and a kilogram a day would be required. At the rate of a 
kilogram a day. a 500,000 to 1,500,000 kilowatt plant would be 
required. (The ultimate capacity of the hydroelectric power 
plants at the Grand Coulee Dam is expected to be 2,000,000 kw.) 
Evidently the creation of a plutonium production plant of the 
required size was to be a major enterprise even without attempt- 
ing to utilize the thermal energy liberated. Nevertheless, by 
November 1942 most of the problems had been well defined and 
tentative solutions had been proposed. Although these problems 
will be discussed in some detail in the next chapter, we will 
mention them here. 

6.42. Since a large amount of heat is generated in any pile 
producing appreciable amounts of plutonium, the first problem 

Metallurgical Project in 1942 105 

of design is a cooling system. Before such a system can be de- 
signed, it is necessary to find the maximum temperature at which 
a pile can run safely and the factors nuclear or structural 
\\ hich determine this temperature. Another major problem is the 
method for loading and unloading the uranium, a problem com- 
plicated by the shielding and the cooling system. Shielding against 
radiation has to be planned for both the pile itself and the chemi- 
cal separation plant. The nature of the separation plant depends 
on the particular separation process to be used, which has to be 
decided. Finally, speed of procurement and construction must 
be primary factors in the planning of both the pile and the 
chemical plant. 


6.43. After examining the principal factors affecting plant 
design, i.e., cooling, efficiency, safety, and speed of construction, 
the "Feasibility Report" suggested a number of possible plant 
types in the following order of preference: 

I. (a) Ordinary uranium metal lattice in a graphite moder- 
ator with helium cooling, (b) The same, with water 
cooling, (c) The same, with molten bismuth cooling. 
II. Ordinary uranium metal lattice in a heavy-water moder- 

III. Uranium enriched in the 235 isotope using graphite, 
heavy water, or ordinary water as moderator. 

Types II and III were of no immediate interest since neither 
enriched uranium nor heavy water was available. Development 
of both these types continued however, since if no other type 
proved feasible they might have to be used. Type I (c), calling 
for liquid bismuth cooling, seemed very promising from the point 
of view of utilization of the thermal energy released, but it was 
felt that the technical problems involved could not be solved for 
a long time. 

106 Metallurgical Project in 1942 


6.44. During this period, the latter half of 1942, when pro- 
duction plants were being planned, it was recognized that a plant 
of intermediate size was desirable. Such a plant was needed for 
two reasons: first, as a pilot plant; second, as a producer of a few 
grams of plutonium badly needed for experimental purposes. 
Designed as an air-cooled plant of 1,000-kw capacity, the inter- 
mediate pile constructed at Clinton, Tennessee, might have 
served both purposes if helium cooling had been retained for the 
main plant. Although the plans for the main plant were shifted 
so that water cooling was called for, the pilot plant was continued 
with air cooling in the belief that the second objective would be 
reached more quickly. It thus ceased to be a pilot plant except 
for chemical separation. Actually the main plant was built 
without benefit of a true pilot plant, much as if the hydroelectric 
generators at Grand Coulee had been designed merely from 
experience gained with a generator of quite different type and 
of a small fraction of the power. 


6.45. As reviewed by Hilberry in the "Feasibility Report" of 
November 26, 1 942, the prospects for a graphite pile with helium 
cooling looked promising as regards immediate production; the 
pile using heavy water for moderator and using heavy water or 
ordinary water as coolant looked better for eventual full-scale 
use. A number of specific proposals were made for construction 
of such plants and for the further study of the problems involved. 
These proposals were based on time and cost estimates which were 
necessarily little better than rough guesses. As the result of further 
investigation the actual program of construction described in 
later chapters has been quite different from that proposed. 


6.46. The procurement problem which had been delaying 
progress was essentially solved by the end of 1942. A small self- 

Metallurgical Project in 1942 107 

sustaining graphite-uranium pile was constructed in November 
1942, and was put into operation for the first time on December 2, 
1942, at a power level of }/ watt and later at 200 watts. It was 
easily controllable thanks to the phenomenon of delayed neutron 
emission. A total of 500 micrograms of plutonium was made with 
the cyclotron and separated chemically from the uranium and 
fission products. Enough was learned of the chemistry of plu- 
tonium to indicate the possibility of separation on a relatively 
large scale. No great advance was made on bomb theory, but 
calculations were checked and experiments with fast neutrons 
extended. If anything, the bomb prospects looked more favorable 
than a year earlier. 

6.47. Enough experimenting and planning were done to 
delineate the problems to be encountered in constructing and 
operating a large-scale production plant. Some progress was 
made in choice of type of plant, first choice at that time being a 
pile of metallic uranium and graphite, cooled either by helium 
or water. A specific program was drawn up for the construction 
of pilot and production plants. This program presented time and 
cost estimates. 




7.1. By the first of January 1943, the Metallurgical Laboratory 
had achieved its first objective, a chain-reacting pile, and was 
well on the way to the second, a process for extracting the 
plutonium produced in such a pile. It was clearly time to formu- 
late more definite plans for a production plant. The policy de- 
cisions were made by the Policy Committee (see Chapter V) on 
the recommendations from the Laboratory Director (A. H. 
Compton), from the S-l Executive Committee, and from the 
Reviewing Committee that had visited Chicago in December 
1942. The only decisions that had already been made were that 
the first chain-reacting pile should be dismantled and then 
reconstructed on a site a short distance from Chicago and that 
a 1,000-kilowatt plutonium plant should be built at Clinton, 


7.2. The first decision to be made was on the scale of pro- 
duction that should be attempted. For reasons of security the 
figure decided upon may not be disclosed here. It was very large. 


7.3. As we have seen, the production of one gram of plutonium 
per day corresponds to a generation of energy at the rate of 
500 to 1,500 kilowatts. Therefore a plant for large-scale produc- 
tion of plutonium will release a very large amount of energy. 


Plutonium Problem February 1943 109 

The problem therefore was to design a plant of this capacity on 
the basis of experience with a pile that could operate at a power 
level of only 0.2 kilowatt. As regards the plutonium separation 
work, which was equally important, it was necessary to draw 
plans for an extraction and purification plant which would 
separate some grams a day of plutonium from some tons of 
uranium, and such planning had to be based on information 
obtained by microchemical studies involving only half a milli- 
gram of plutonium. To be sure, there w*as information available 
for the design of the large-scale pile and separation plant from 
auxiliary experiments and from large-scale studies of separation 
processes using uranium as a stand-in for plutonium, but even 
so the proposed extrapolations both as to chain-reacting piles 
and as to separation processes were staggering. In peacetime no 
engineer or scientist in his right mind would consider making 
such a magnification in a single stage, and even in wartime only 
the possibility of achieving tremendously important results 
could justify it. 


7.4. As soon as it had been decided to go ahead with large- 
scale production of plutonium, it was evident that a great ex- 
pansion in organization was necessary. The Stone and Webster 
Engineering Corporation had been selected as the overall engi- 
neering and construction firm for the DSM Project soon after 
the Manhattan District was placed in charge of construction 
work in June 1942. By October 1942, it became evident that 
various component parts of the work were too far separated 
physically and were too complicated technically to be handled 
by a single company especially in view of the rapid pace re- 
quired. Therefore it was decided that it would be advantageous 
if Stone and Webster were relieved of that portion of the work 
pertaining to the construction of plutonium production facilities. 
This was done, and General Groves selected the E. I. du Pont 
de Nemours and Company as the firm best able to carry on this 
phase of the work. The arrangements made with various indus- 

110 Plutonium Problem February 1943 

trial companies by the Manhattan District took various forms. 
The arrangements with du Pont are discussed in detail as an 

7.5. General Groves broached the question to W. S. Carpenter, 
Jr., president of du Pont, and after considerable discussion with 
him and other officials of the firm, du Pont agreed to undertake 
the work. In their acceptance, they made it plain and it was 
understood by all concerned that du Pont was undertaking the 
work only because the War Department considered the work to 
be of the utmost importance, and because General Groves stated 
that this view as to importance was one held personally by the 
President of the United States, the Secretary of War, the Chief 
of Staff, and General Groves, and because of General Groves' 
assertion that du Pont was by far the organization best qualified 
for the job. At the same time, it was recognized that the du Pont 
Company already had assumed all the war-connected activities 
which their existing organization could be expected to handle 
without undue difficulty. 

7.6. The du Pont Company, in accepting the undertaking, 
insisted that the work be conducted without profit and without 
patent rights of any kind accruing to them. The du Pont Com- 
pany did request, however, that in view of the unknown character 
of the field into which they were being asked to embark, and in 
view of the unpredictable hazards involved, the Government 
provide maximum protection against losses sustained by du Pont. 

7.7. The cost-plus-a-fixed-fee contract between the Govern- 
ment and du Pont established a fixed fee of SI. 00. The Govern- 
ment agreed to pay all costs of the work by direct reimbursement 
or through allowances provided by the contract to cover ad- 
ministrative and general expenses allocated to the work in accord- 
ance with normal du Pont accounting practices as determined by 
audit by certified public accountants. Under the terms of the 
contract, any portion of these allowances not actually expended 
by du Pont will, at the conclusion of the work, be returned to the 
United States. The contract also provided that no patent rights 
would accrue to the company. 

Plutonium Problem February 1943 111 

7.8. The specific responsibilities assumed by du Pont were to 
engineer, design, and construct a small-scale semi- works at the 
Clinton Engineer Works in Tennessee and to engineer, design, 
construct, and operate a large-scale plutonium production plant 
of large capacity at the Hanford Engineer Works in the State 
of Washington. Because of its close connection with fundamental 
research, the Clinton semi-works was to be operated under the 
direction of the University of Chicago. A large number of key 
technical people from du Pont were to be used on a loan basis at 
Chicago and at Clinton, to provide the University with much 
needed personnel, particularly men with industrial experience, 
and to train certain of such personnel for future service at Hanford. 

7.9. Inasmuch as du Pont was being asked to step out of its 
normal role in chemistry into a new field involving nuclear 
physics, it was agreed that it would be necessary for them to 
depend most heavily upon the Metallurgical Laboratory of the 
University of Chicago for fundamental research and development 
data and for advice. The du Pont Company had engineering and 
industrial experience, but it needed the Metallurgical Laboratory 
for nuclear-physics and radiochemistry experience. The Metal- 
lurgical Laboratory conducted the fundamental research on 
problems bearing on the design and operation of the semi-works 
and large-scale production plants. It proposed the essential parts 
of the plutonium production and recovery processes and equip- 
ment, answered the many specific questions raised by du Pont, 
and studied and concurred in the final du Pont decisions and 

7.10. The principal purpose of the Clinton semi- works was 
development of methods of operation for plutonium recovery. 
The semi-works had to include of course, a unit for plutonium 
production, in order to provide plutonium to be recovered experi- 
mentally. In the time and with the information available, the 
Clinton production unit could not be designed to be an early 
edition of the Hanford production units which, therefore, had to 
be designed, constructed and operated without major guidance 
from Clinton experience. In fact, even the Hanford recover}' 

112 Plutonium Problem February 1943 

units had to be far along in design and procurement of equip- 
ment before Clinton results became available. However, the 
Clinton semi-works proved to be an extremely important tool in 
the solution of the many completely new problems encountered 
at Hanford. It also produced small quantities of plutonium 
which, along with Metallurgical Laboratory data on the prop- 
erties of plutonium, enabled research in the use of this material 
to be advanced many months. 


7.11. Once the scale of production had been agreed upon 
and the responsibilities assigned, the nature of the plant and its 
whereabouts had to be decided. The site in the Tennessee Valley, 
known officially as the Clinton Engineer Works, had been ac- 
quired by the Army for the whole program as recommended in 
the report to the President (see Chapter V). 

7.12. Reconsideration at the end of 1942 led General Groves 
to the conclusion that this site was not sufficiently isolated for a 
large-scale plutonium production plant. At that time, it was 
conceivable that conditions might arise under which a large pile 
might spread radioactive material over a large enough area to 
endanger neighboring centers of population. In addition to the 
requirement of isolation, there remained the requirement of a 
large power supply which had originally determined the choice 
of the Tennessee site. To meet these two requirements a new site 
was chosen and acquired on the Columbia River in the central 
part of the State of Washington near the Grand Coulee power 
line. This site was known as the Hanford Engineer Works. 

7.13. Since the Columbia River is the finest supply of pure 
cold river water in this country, the Hanford site was well suited 
to either the helium-cooled plant originally planned or to the 
water-cooled plant actually erected. The great distances sepa- 
rating the home office of du Pont in Wilmington, Delaware, the 
pilot plant at Clinton, Tennessee, the Metallurgical Laboratory 
at Chicago, and the Hanford site were extremely inconvenient, 
but this separation could not be avoided. Difficulties also were 

Plutonium Problem February 1943 113 

inherent in bringing workmen to the site and in providing living 
accommodations for them. 


7.14. It was really too early in the development to make a 
carefully weighed decision as to the best type of plutonium pro- 
duction plant. Yet a choice had to be made so that design could 
be started and construction begun as soon as possible. Actually 
a tentative choice was made and then changed. 

7.15. In November 1942, the helium-cooled plant was the 
first choice of the Metallurgical Laboratory. Under the direction 
of T. Moore and M. C. Leverett, preliminary plans for such 
a plant had been worked out. The associated design studies were 
used as bases for choice of site, choice of accessory equipment, 
etc. Although these studies had been undertaken partly because 
it had been felt that they could be carried through more quickly 
for a helium-cooled plant than for a water-cooled plant, many 
difficulties were recognized. Meanwhile the theoretical group 
under Wigner, with the cooperation of the engineering personnel, 
had been asked to prepare a report on a water-cooled plant of 
high power output. This group had been interested in water- 
cooling almost from the beginning of the project and was able to 
incorporate the results of its studies in a report issued on January 
9, 1943. This report contained many important ideas that were 
incorporated in the design of the production plant erected at 

7.16. When du Pont came into the picture, it at first accepted 
the proposal of a helium-cooled plant but after further study 
decided in favor of water cooling. The reasons for the change 
were numerous. Those most often mentioned were the hazard 
from leakage of a high-pressure gas coolant carrying radioactive 
impurities, the difficulty of getting large blowers quickly, the 
large amount of helium required, the difficulty of loading and 
unloading uranium from the pile, and the relatively low power 
output per kilogram of uranium metal. These considerations 
had to be balanced against the peculiar disadvantages of a water- 

114 Plutonium Problem February 1943 

cooled plant, principally the greater complexity of the pile itself 
and the dangers of corrosion. 

7.17. Like so many decisions in this project, the choice between 
various types of plant had to be based on incomplete scientific 
information. The information is still incomplete, but there is 
general agreement that water cooling was the wise choice. 



7.18. In Chapter II of this report we attempted to define 
the general problem of the uranium project as it appeared in the 
summer of 1940. We now wish to give precise definition to the 
problem of the design of a large-scale plant for the production 
of plutonium. The objective had already been delimited by 
decisions as to scale of production, type of plant, and site. As it 
then stood, the specific problem was to design a water-cooled 
graphite-moderated pile (or several such piles) with associated 
chemical separation plant to produce a specified, relatively large 
amount of plutonium each day, the plant to be built at the Han- 
ford site beside the Columbia River. Needless to say, speed of 
construction and efficiency of operation were prime considerations. 


7.19. The lattices we have been describing heretofore con- 
sisted of lumps of uranium imbedded in the graphite moderator. 
There are two objections to such a type of lattice for production 
purposes: first, it is difficult to remove the uranium without dis- 
assembling the pile; second, it is difficult to concentrate the 
coolant at the uranium lumps, which are the points of maximum 
production of heat. It was fairly obvious that both these diffi- 
culties could be avoided if a rod lattice rather than a point lattice 
could be used, that is, if the uranium could be concentrated 
along lines passing through the moderator instead of being situated 
merely at points. There was little doubt that the rod arrange- 
ment would be excellent structurally and mechanically, but 

Plutonium Problem February 1943 115 

there was real doubt as to whether it was possible to build such 
a lattice which would still have a multiplication factor k greater 
than unity. This became a problem for both the theoretical and 
experimental physicists. The theoretical physicists had to compute 
what was the optimum spacing and diameter of uranium rods; 
the experimental physicists had to perform exponential experi- 
ments on lattices of this type in order to check the findings of 
the theoretical group. 


7.20. Once the idea of a lattice with cylindrical symmetry 
was accepted, it became evident that the pile could be unloaded 
and reloaded without disassembly since the uranium could be 
pushed out of the cylindrical channels in the graphite moderator 
and new uranium inserted. The decision had to be made as to 
whether the uranium should be in the form of long rods, which 
had advantages from the nuclear-physics point of view, or of 
relatively short cylindrical pieces, which had advantages from 
the point of view of handling. In either case, the materials would 
be so very highly radioactive that unloading would have to be 
carried out by remote control, and the unloaded uranium would 
have to be handled by remote control from behind shielding. 


7.21. If water was to be used as coolant, it would have to be 
conveyed to the regions where heat was generated through 
channels of some sort. Since graphite pipes were not practical, 
some other kind of pipe would have to be used. But the choice 
of the material for the pipe, like the choice of all the materials 
to be used hi the pile, was limited by nuclear-physics considera- 
tions. The pipes must be made of some material whose absorption 
cross section for neutrons was not large enough to bring the value 
of k below unity. Furthermore, the pipes must be made of mate- 
rial which would not disintegrate under the heavy density of 
neutron and gamma radiation present in the pile. Finally, the 
pipes must meet all ordinary requirements of cooling-system 

116 Plutonium Problem February 1943 

pipes: they must not leak; they must not corrode; they must not 

7.22. From the nuclear-physics point of view there were seven 
possible materials (Pb, Bi, Be, Al, Mg, Zn, Sn), none of which 
had high neutron-absorption cross sections. No beryllium tubing 
was available, and of all the other metals only aluminum was 
thought to be possible from a corrosion point of view. But it 
was by no means certain that aluminum would be satisfactory, 
and doubts about the corrosion of the aluminum pipe were not 
settled until the plant had actually operated for some time. 

7.23. While the choice of material for the piping was very 
difficult, similar choices involving both nuclear-physics criteria 
and radiation-resistance criteria had to be made for all other 
materials that were to be used in the pile. For example, the elec- 
tric insulating materials to be used in any instruments buried 
in the pile must not disintegrate under the radiation. In certain 
instances where control or experimental probes had to be in- 
serted and removed from the pile, the likelihood had to be borne 
in mind that the probes would become intensely radioactive as 
a result of their exposure in the pile and that the degree to which 
this would occur would depend on the material used. 

7.24. Finally, it was not known what effect the radiation fields 
in the pile would have on the graphite and the uranium. It 
was later found that the electric resistance, the elasticity, and the 
heat conductivity of the graphite all change with exposure to 
intense neutron radiation. 


7.25. The most efficient cooling procedure would have been 
to have the water flowing in direct contact with the uranium in 
which the heat was being produced. Indications were that this 
was probably out of the question because the uranium would 
react chemically with the water, at least to a sufficient extent to 
put a dangerous amount of radioactive material into solution 
and probably to the point of disintegrating the uranium slugs. 
Therefore it was necessary to find some method of protecting the 

Plutonium Problem February 1943 117 

uranium from direct contact with the water. Two possibilities 
were considered: one was some sort of coating, either by electro- 
plating or dipping; the other was sealing the uranium slug in a 
protective jacket or "can." Strangely enough, this "canning 
problem" turned out to be one of the most difficult problems 
encountered in such piles. 


7.26. The problem of dissipating thousands of kilowatts of 
energy is by no means a small one. How much water was needed 
depended, of course, on the maximum temperature to which the 
water could safely be heated and the maximum temperature to 
be expected in the intake from the Columbia River; certainly 
the water supply requirement was comparable to that of a fair- 
sized city. Pumping stations, filtration and treatment plants all 
had to be provided. Furthermore, the system had to be a very 
reliable one; it was necessary to provide fast-operating controls 
to shut down the chain-reacting unit in a hurry in case of failure 
of the water supply. If it was decided to use "once-through" 
cooling instead of recirculation, a retention basin would be 
required so that the radioactivity induced in the water might 
die down before the water was returned to the river. The volume 
of water discharged was going to be so great that such problems 
of radioactivity were important, and therefore the minimum 
time that the water must be held for absolute safety had to be 


7.27. The control problem was very similar to that discussed in 
connection with the first chain-reacting pile except that every- 
thing was on a larger scale and was, therefore, potentially more 
dangerous. It was necessary to provide operating controls which 
would automatically keep the pile operating at a determined 
power level. Such controls had to be connected with instruments 
in the pile which would measure neutron density or some other 
property which indicated the power level. There would also have 

118 Plutonium Problem February 1943 

to be emergency controls which would operate almost instan- 
taneously if the power level showed signs of rapid increase or if 
there was any interruption of the water supply. It was highly 
desirable that there be some means of detecting incipient diffi- 
culties such as the plugging of a single water tube or a break in 
the coating of one of the uranium slugs. All these controls and 
instruments had to be operated from behind the thick shielding 
walls described below. 


7.28. As we have mentioned a number of times, the radiation 
given off from a pile operating at a high power level is so strong 
as to make it quite impossible for any of the operating personnel 
to go near the pile. Furthermore, this radiation, particularly the 
neutrons, has a pronounced capacity for leaking out through 
holes or cracks in barriers. The whole of a power pile therefore 
has to be enclosed in very thick walls of concrete, steel, or other 
absorbing material. But at the same time it has to be possible to 
load and unload the pile through these shields and to carry the 
water supply in and out through the shields. The shields should 
not only be radiation-tight but air-tight since air exposed to the 
radiation in the pile would become radioactive. 

7.29. The radiation dangers that require shielding in the pile 
continue through a large part of the separation plant. Since the 
fission products associated with the production of the plutonium 
are highly radioactive, the uranium after ejection from the pile 
must be handled by remote control from behind shielding and 
must be shielded during transportation to the separation plant. 
All the stages of the separation plant, including analyses, must 
be handled by remote control from behind shields up to the 
point where the plutonium is relatively free of radioactive fission 


7.30. The problem of maintenance is very simply stated. There 
could not be any maintenance inside the shield or pile once the 

Plutonium Problem February 1943 119 

pile had operated. The same remark applies to a somewhat lesser 
extent to the separation unit, where it was probable that a shut- 
down for servicing could be effected, provided, of course, that 
adequate remotely-controlled decontamination processes were 
carried out in order to reduce the radiation intensity below the 
level dangerous to personnel. The maintenance problem for the 
auxiliary parts of the plant was normal except for the extreme 
importance of having stand-by pumping and power equipment 
to prevent a sudden accidental breakdown of the cooling system. 


7.31. Evidently the amount of plutonium in an undisturbed 
operating pile increases with time of operation. Since Pu-239 
itself undergoes fission its formation tends to maintain the chain 
reaction, while the gradual disappearance of the U-235 and the 
appearance of fission products with large neutron absorption 
cross sections tend to stop the reaction. The determination of 
when a producing pile should be shut down and the plutonium 
extracted involves a nice balancing of these factors against tune 
schedules, material costs, separation-process efficiency, etc. 
Strictly speaking, this problem is one of operation rather than 
of design of the plant, but some thought had to be given to it in 
order to plan the flow of uranium slugs to the pile and from the 
pile to the separation plant. 


7.32. We have been speaking of the production capacity of the 
plant only in terms of overall production rate. Naturally, a given 
rate of production might be achieved in a single large pile or in 
a number of smaller ones. The principal advantage of the smaller 
piles would be the reduction in construction time for the first 
pile, the possibility of making alterations hi later piles, and 
perhaps most important the improbability of simultaneous 
breakdown of all piles. The disadvantage of small piles is that 
they require disproportionately large amounts of uranium, 

120 Plutonium Problem February 1943 

moderator, etc. There is, in fact, a preferred "natural size" of 
pile which can be roughly determined on theoretical grounds. 


7.33. As we have already pointed out, the slugs coming from 
the pile are highly radioactive and therefore must be processed 
by remote control in shielded compartments. The general scheme 
to be followed was suggested in the latter part of 1 942, particularly 
in connection with plans for the Clinton separation plant. This 
scheme was to build a "canyon" which would consist of a series 
of compartments with heavy concrete walls arranged in a line 
and almost completely buried in the ground. Each compartment 
would contain the necessary dissolving or precipitating tanks or 
centrifuges. The slugs would come into the compartment at one 
end of the canyon; they would then be dissolved and go through 
the various stages of solution, precipitation, oxidation, or reduc- 
tion, being pumped from one compartment to the next until a 
solution of plutonium free from uranium and fission products 
came out in the last compartment. As in the case of the pile, 
everything would be operated by remote control from above 
ground, but the operations would be far more complicated than 
in the case of the pile. However, as far as the chemical operations 
themselves were concerned, their general nature was not so far 
removed from the normal fields of activity of the chemists 


7.34. In the first stages of the separation process even the 
routine analysis of samples which was necessary in checking the 
operation of the various chemical processes had to be done by 
remote control. Such testing was facilitated, however, by use of 
radioactive methods of analysis as well as conventional chemical 


7.35. The raw material (uranium) is not dangerously radio- 
active. The desired product (plutonium) does not give off pene- 

Plutonium Problem February 1943 121 

trating radiation, but the combination of its alpha-ray activity 
and chemical properties makes it one of the most dangerous sub- 
stances known if it once gets into the body. However, the really 
troublesome materials are the fission products, i.e., the major 
fragments into which uranium is split by fission. The fission 
products are very radioactive and include some thirty elements. 
Among them are radioactive xenon and radioactive iodine. 
These are released in considerable quantity when the slugs are 
dissolved and must be disposed of with special care. High stacks 
must be built which will carry off these gases along with the acid 
fumes from the first dissolving unit, and it must be established 
that the mixing of the radioactive gases with the atmosphere will 
not endanger the surrounding territory. (As in all other matters 
of health, the tolerance standards that were set and met were so 
rigid as to leave not the slightest probability of danger to the 
health of the community or operating personnel.) 

7.36. Most of the other fission products can be retained in 
solution but must eventually be disposed of. Of course, possible 
pollution of the adjacent river must be considered. (In fact, the 
standards of safety set and met with regard to river pollution 
were so strict that neither people nor fish down the river can 
possibly be affected.) 


7.37. Evidently, even if the uranium were left in the pile until 
all the U-235 had undergone fission, there would still be a large 
amount of U-238 which had not been converted to plutonium. 
Actually the process is stopped long before this stage is reached. 
Uranium is an expensive material and the total available supply 
is seriously limited. Therefore the possibility of recovering it after 
the plutonium is separated must be considered. Originally there 
was no plan for recovery, but merely the intention of storing 
the uranium solution. Later, methods of large-scale recovery 
were developed 

122 Plutonium Problem February 1943 


7.38. An unusual feature of the chemical processes involved 
was that these processes occur in the presence of a high density 
of radiation. Therefore the containers used may corrode more 
rapidly than they would under normal circumstances. Further- 
more, any such corrosion will be serious because of the difficulty 
of access. For a long time, information was sadly lacking on these 


7.39. The chemical reactions proposed for an extraction 
process were, of course, tested in the laboratory. However, they 
could not be tested with appreciable amounts of plutonium nor 
could they be tested in the presence of radiation of anything like 
the expected intensity. Therefore it was realized that a process 
found to be successful in the laboratory might not work in the 


7.40. The description given above as to what was to happen 
in the successive chambers in the canyon was very vague. This 
was necessarily so, since even by January 1943 no decision had 
been made as to what process would be used for the extraction 
and purification of plutonium. The major problem before the 
Chemistry Division of the Metallurgical Laboratory was the 
selection of the best process for the plant. 


7.41. Besides the hazards normally present during construction 
and operation of a large chemical plant, dangers of a new kind 
were expected here. Two types of radiation hazard were antici- 
pated neutrons generated in the pile, and alpha particles, 
beta particles, and gamma rays emitted by products of the pile. 
Although the general effects of these radiations had been proved 

Plutonium Problem February 1943 123 

to be similar to those of X-rays, very little detailed knowledge 
was available. Obviously the amounts of radioactive material 
to be handled were many times greater than had ever been en- 
countered before. 

7.42. The health group had to plan three programs: (1) pro- 
vision of instruments and clinical tests to detect any evidence of 
dangerous exposure of the personnel; (2) research on the effects 
of radiation on persons, instruments, etc.; and (3) estimates of 
what shielding and safety measures must be incorporated in the 
design and plan of operation of the plant. 


7.43. Although we were embarking on a major enterprise to 
produce plutonium, we still had less than a milligram to study 
and still had only limited familiarity with its properties. The 
study of plutonium, therefore, remained a major problem of the 
Metallurgical Laboratory. 


7.44. Evidently the operation of a full-scale plant of the type 
planned would require a large and highly skilled group of oper- 
ators. Although du Pont had a tremendous background of 
experience in the operation of various kinds of chemical plant, 
this was something new and it was evident that operating per- 
sonnel would need special training. Such training was carried 
out partly in Chicago and its environs, but principally at the 
Clinton Laboratories. 


7.45. In the preceding paragraphs of this chapter we have 
outlined the problems confronting the group charged with 
designing and building a plutonium production plant. In Chapter 
VI the progress in this field up to the end of 1942 was reviewed. 
Throughout these chapters it is made clear that a great deal more 

124 Plutonium Problem February 1943 

information was required to assure the success of the plant. 
Such answers as had been obtained to most of the questions were 
only tentative. Consequently research had to be pushed simul- 
taneously with planning and construction. 


7.46. To meet the need for further information, research pro- 
grams were laid out for the Metallurgical Laboratory and the 
Clinton Laboratory. The following passage is an excerpt from 
the 1943 program of the Metallurgical Project: 

"Product Production Studies. These include all aspects of the re- 
search, development and semi-works studies necessary for the 
design, construction, and operation of chain-reacting piles to 
produce plutonium or other materials. 

Pile Characteristics. Theoretical studies and experiments on 
lattice structures to predict behavior in high-level piles, such 
as temperature and barometric effects, neutron character- 
istics, pile poisoning, etc. 

Control of Reacting Units. Design and experimental tests of 
devices for controlling rate of reaction in piles. 
Cooling of Reacting Units. Physical studies of coolant material, 
engineering problems of circulation, corrosion, erosion, etc. 
Instrumentation. Development of instruments and technique 
for monitoring pile and surveying radiation throughout 
plant area. 

Protection. Shielding, biological effects of radiation at pile 
and clinical effects of operations associated with pile. 
Materials. Study of physical (mechanical and nuclear) prop- 
erties of construction and process materials used in pile 
construction and operation. 

Activation Investigations. Production of experimental amounts 
of radioactive materials in cyclotron and in piles and study 
of activation of materials by neutrons, protons, electrons, 
gamma rays, etc. 

Plutonium Problem February 1943 125 

Pile Operation. Study of pile operation procedures such as 
materials handling, instrument operation, etc. 
Process Design. Study of possible production processes as a 
whole leading to detailed work in other categories. 

"Product Recovery Studies. These include all aspects of the work 
necessary for the development of processes for the extraction of 
plutonium and possible by-products from the pile material and 
their preparation in purified form. Major effort at the Metal- 
lurgical Laboratory will be on a single process to be selected by 
June 1, 1943 for the production of plutonium, but alternatives 
will continue to be studied both at the Metallurgical Laboratory 
and Clinton with whatever manpower is available. 

Separation. Processes for solution of uranium, extraction of 
plutonium and decontamination by removal of fission 

Concentration, Purification and Product Reduction. Processes lead- 
ing to production of plutonium as pure metal, and study of 
properties of plutonium necessary to its production. 
Wastes. Disposal and possible methods of recovery of fission 
products and metal from wastes. 

Instrumentation. Development and testing of instruments for 
monitoring chemical processes and surveying radiation 
throughout the area. 

Protection. Shi-elding studies, determination of biological 
effects of radioactive dusts, liquids, solids, and other process 
materials, and protective measures. 

Materials. Corrosion of equipment materials, and radiation 
stability. Necessary purity and purity analysis of process 
materials, etc. 

Recovery of Activated Materials. Development of methods and 
actual recovery of activated material (tracers, etc.) from 
cyclotron and pile-activated materials. 

126 Plutonium Problem February 1943 

Operations Studies. Equipment performance, process control, 
material handling operations, etc. 

Process Design. Study of product recovery processes as a 
whole (wet processes, physical methods) leading to detailed 
work in other categories. 

"Fundamental Research. Studies of the fundamental physical, 
chemical and biological phenomena occurring in chain-reacting 
piles, and basic properties of all materials involved. Although 
the primary emphasis at Clinton is on the semi-works level, 
much fundamental research will require Clinton conditions (high 
radiation intensity, large scale processes). 

Nuclear Physics. Fundamental properties of nuclear fission 
such as cross section, neutron yield, fission species, etc. 
Other nuclear properties important to processes, such as 
cross sections, properties of moderators, neutron effect on 
materials, etc. 

General Physics. Basic instrument (electronic, ionization, 
optical, etc.) research, atomic mass determinations, neutron, 
a, /3, 7 radiation studies, X-ray investigations, etc. 
Radiation Chemistry. Effects of radiation on chemical proc- 
esses and chemical reactions produced by radiation. 
Nuclear Chemistry. Tracing of fission products, disintegration 
constants, chains, investigation of nuclei of possible use to 

Product Chemistry. Chemical properties of various products 
and basic studies in separation and purification of products. 
General Chemistry. Chemistry of primary materials and mate- 
rials associated with process, including by-products. 
General Biology. Fundamental studies of effects of radiation 
on living matter, metabolism of important materials, etc. 
Clinical Investigations. Basic investigations, such as hema- 
tology, pathology, etc. 
Metallurgical Studies. Properties of U, Pu, Be, etc. 

Plutonium Problem February 1943 127 

Engineering Studies. Phenomena basic to corrosion and similar 
studies essential to continued engineering development of 

7.47. An examination of this program gives an idea of the 
great range of investigations which were considered likely to 
give relevant information. Many of the topics listed are not 
specific research problems such as might be solved by a small 
team of scientists working for a few months but are whole fields 
of investigation that might be studied with profit for years. It was 
necessary to pick the specific problems that were likely to give 
the most immediately useful results but at the same time it was 
desirable to try to uncover general principles. For example, the 
effect of radiation on the properties of materials ("radiation 
stability") was almost entirely unknown. It was necessary both 
to make empirical tests on particular materials that might be 
used in a pile and to devise general theories of the observed 
effects. Every effort was made to relate all work to the general 
objective: a successful production plant. 


7.48. There have been many changes in the organization and 
personnel of the project. During most of the period of construc- 
tion at Clinton and Hanford, A. H. Compton was director of the 
Metallurgical Project; S. K. Allison was director of the Metal- 
lurgical Laboratory at Chicago; and M. D. Whitaker was direc- 
tor of the Clinton Laboratory. The Chicago group was organized 
in four divisions: physics, chemistry, technology, and health. 
Later the Physics Division was split into general physics and 
nuclear physics. R. L. Doan was research director at Clinton 
but there was no corresponding position at Chicago. Among 
others who have been associate or assistant laboratory or project 
directors or have been division directors are S. T. Cantril, C. M. 
Cooper, F. Daniels, A. J. Dempster, E. Fermi, J. Franck, 
N. Hilberry, T. R. Hogness, W. C. Johnson, H. D. Smyth, J. C. 
Stearns, R. S. Stone, H. C. Vernon, W. W. Watson, and E. 

128 Plutonium Problem February 1943 

Wigner. Beginning in 1943 C. H. Thomas of the Monsanto 
Chemical Company acted as chairman of a committee on the 
Chemistry and Metallurgy of Plutonium. This committee cor- 
related the activities of the Metallurgical Laboratory with those 
at Los Alamos (see Chapter XII) and elsewhere. Later the 
Monsanto Chemical Company did some work on important 
special problems arising in connection with the Los Alamos work. 

7.49. It was the responsibility of these men to see that the 
research program described above was carried out and that 
significant results were reported to du Pont. It was their responsi- 
bility also to answer questions raised by du Pont and to approve 
or criticize plans submitted by du Pont. 


7.50. Since du Pont was the design and construction organiza- 
tion and the Metallurgical Laboratory was the research organiza- 
tion, it was obvious that close cooperation was essential. Not only 
did du Pont need answers to specific questions, but they could 
benefit by criticism and suggestions on the many points where 
the Metallurgical group was especially well-informed. Similarly, 
the Metallurgical group could profit by the knowledge of du Pont 
on many technical questions of design, construction, and opera- 
tion. To promote this kind of cooperation du Pont stationed one 
of their physicists, J. B. Miles, at Chicago, and had many other 
du Pont men, particularly C. H. Greenewalt, spend much of 
their time at Chicago. Miles and Greenewalt regularly attended 
meetings of the Laboratory Council. There was no similar 
reciprocal arrangement although many members of the labora- 
tory visited Wilmington informally. In addition, J. A. Wheeler 
was transferred from Chicago to Wilmington and became a 
member .of the du Pont staff. There was, of course, constant 
exchange of reports and letters, and conferences were held fre- 
quently between Compton and R. Williams of du Pont. Whitaker 
spent much of his time at Wilmington during the period when the 
Clinton plant was being designed and constructed. 

Plutonium Problem February 1943 129 


7.51. By January 1943, the decision had been made to build 
a plutonium production plant with a large capacity. This meant 
a pile developing thousands of kilowatts and a chemical separa- 
tion plant to extract the product. The du Pont Company was to 
design, construct, and operate the plant; the Metallurgical Labor- 
atory was to do the necessary research. A site was chosen on the 
Columbia River at Hanford, Washington. A tentative decision 
to build a helium-cooled plant was reversed in favor of water- 
cooling. The principal problems were those involving lattice 
design, loading and unloading, choice of materials particularly 
with reference to corrosion and radiation, water supply, controls 
and instrumentation, health hazards, chemical separation process, 
and design of the separation plant. Plans were made for the neces- 
sary fundamental and technical research and for the training of 
operators. Arrangements were made for liaison between du Pont 
and the Metallurgical Laboratory. 

JANUARY 1943 TO JUNE 1945 


8.1. The necessity for pushing the design and construction of 
the full-scale plutonium plant simultaneously with research and 
development inevitably led to a certain amount of confusion 
and inefficiency. It became essential to investigate many alter- 
native processes. It became necessary to investigate all possible 
causes of failure even when the probability of their becoming 
serious was very small. Now that the Hanford plant is producing 
plutonium successfully, we believe it is fair to say that a large 
percentage of the results of investigation made between the end 
of 1942 and the end of 1944 will never be used at least not for 
the originally intended purposes. Nevertheless had the Hanford 
plant run into difficulties, any one of the now superfluous in- 
vestigations might have furnished just the information required 
to convert failure into success. Even now it is impossible to say 
that future improvements may not depend on the results of 
researches that seem unimportant today. 

8.2. It is estimated that thirty volumes will be required for a 
complete report of the significant scientific results of researches 
conducted under the auspices of the Metallurgical Project. 
Work was done on every item mentioned on the research program 
presented in the last chapter. In the present account it would be 
obviously impossible to give more than a brief abstract of all 
these researches. We believe this would be unsatisfactory and 
that it is preferable to give a general discussion of the chain- 
reacting units and separation plants as they now operate, with 
some discussion of the earlier developments. 


8.3. In Chapter I and other early chapters we have given 
brief accounts of the fission process, pile operation, and chemical 


Plutonium Problem to June 1945 131 

separation. We shall now review these topics from a somewhat 
different point of view before describing the plutonium produc- 
tion plants themselves. 

8.4. The operation of a pile depends on the passage of neutrons 
through matter and on the nature of the collisions of neutrons 
with the nuclei encountered. The collisions of principal impor- 
tance are the following: 

I. Collisions in which neutrons are scattered and lose appreci- 
able amounts of energy, (a) Inelastic collisions of fast neutrons 
with uranium nuclei, (b) Elastic collisions of fast or moderately 
fast neutrons with the light nuclei of the moderator material; 
these collisions serve to reduce the neutron energy to very low 
(so-called thermal) energies. 

II. Collisions in which the neutrons are absorbed, (a) Colli- 
sions which result in fission of nuclei and give fission products and 
additional neutrons, (b) Collisions which result in the formation 
of new nuclei which subsequently disintegrate radioactively 
(e.g., wU 239 which produces 94 Pu 239 ). 

8.5. Only the second class of collision requires further dis- 
cussion. As regards collisions of Type II (a), the most important 
in a pile are the collisions between neutrons and U-235, but the 
high-energy fission of U-238 and the thermal fission of Pu-239 
also take place. Collisions of Type II (b) are chiefly those between 
neutrons and U-238. Such collisions occur for neutrons of all 
energies, but they are most likely to occur for neutrons whose 
energies lie in the "resonance" region located somewhat above 
thermal energies. The sequence of results of the Type II (b) 
collision is represented as follows: 

M U 238 + on 1 -> 92 U 239 + gamma rays 
92 U 239 ^j 93 Np 239 -f _ ie 

93N P 239 TTdl^ 94Fu239 + - ie + g amma ra ys 

8.6. Any other non-fission absorption processes are important 
chiefly because they waste neutrons; they occur in the moderator, 

132 Plutonium Problem to June 1945 

in U-235, in the coolant, in the impurities originally present, in 
the fission products, and even in plutonium itself. 

8.7. Since the object of the chain reaction is to generate 
plutonium, we would like to absorb all excess neutrons in U-238, 
leaving just enough neutrons to produce fission and thus to 
maintain the chain reaction. Actually the tendency of the 
neutrons to be absorbed by the dominant isotope U-238 is so 
great compared to their tendency to produce fission in the 140- 
times-rarer U-235 that the principal design effort had to be 
directed toward favoring the fission (as by using a moderator, 
a suitable lattice, materials of high purity, etc.,) in order to 
maintain the chain reaction. 


8.8. All the chain-reacting piles designed by the Metallurgical 
Laboratory or with its cooperation consist of four categories of 
material the uranium metal, the moderator, the coolant, and 
the auxiliary materials such as water tubes, casings of uranium, 
control strips or rods, impurities, etc. All the piles depend on 
stray neutrons from spontaneous fission or cosmic rays to initiate 
the reaction. 

8.9. Suppose that the pile were to be started by simultaneous 
release (in the uranium metal) of N high-energy neutrons. 
Most of these neutrons originally have energies above the thresh- 
old energy of fission of U-238. However, as the neutrons pass 
back and forth in the metal and moderator, they suffer numerous 
inelastic collisions with the uranium and numerous elastic colli- 
sions with the moderator, and all these collisions serve to reduce 
the energies below that threshold. Specifically, in a typical 
graphite-moderated pile a neutron that has escaped from the 
uranium into the graphite travels on the average about 2.5 cm 
between collisions and makes on the average about 200 elastic 
collisions before passing from the graphite back into the uranium. 
Since at each such collision a neutron loses on the average about 
one sixth of its energy, a one-Mev neutron is reduced to thermal 

* See drawing facing p. 35. 

Plutonium Problem to June 1945 133 

energy (usually taken to be 0.025 electron volt) considerably 
before completing a single transit through the graphite. There 
are, of course, many neutrons that depart from this average 
behavior, and there will be enough fissions produced by fast 
neutrons to enhance slightly the number of neutrons present. 
The enhancement may be taken into account by multiplying 
the original number of neutrons N by a factor which is called 
the fast-fission effect or the fast-multiplication factor. 

8.10. As the average energy of the N neutrons present con- 
tinues to fall, inelastic collision in the uranium becomes unimpor- 
tant, the energy being reduced essentially only in the moderator. 
However, the chance of non-fission absorption (resonance cap- 
ture) in U-238 becomes significant as the intermediate or reson- 
ance energy region is reached. Actually quite a number of 
neutrons in this energy region will be absorbed regardless of choice 
of lattice design. The effect of such capture may be expressed by 
multiplying N by a factor p, (which is always less than one) 
called the "resonance escape probability" which is the proba- 
bility that a given neutron starting with energy above the reson- 
ance region will reach thermal energies without absorption in 
U-238. Thus from the original N high-energy neutrons we obtain 
Nep neutrons of thermal energy. 

8.11. Once a neutron has reached thermal energy the chance 
that it will lose more energy by collision is no greater than the 
chance that it will gain energy. Consequently the neutrons will 
remain at this average energy until they are absorbed. In the 
thermal-energy region the chance for absorption of the neutron 
by the moderator, the coolant and the auxiliary materials is 
greater than at higher energies. At any rate it is found that we 
introduce little error into our calculations by assuming all such 
unwanted absorption takes place in this energy region. We now 
introduce a factor f, called the thermal utilization factor, which 
is defined as the probability that a given thermal neutron will be 
absorbed in the uranium. Thus from the original N fast neutrons 
we have obtained Ntpf thermal neutrons which are absorbed by 

134 Plutonium Problem to June 1945 

8.12. Although there are several ways in which the normal 
mixture of uranium isotopes can absorb neutrons, the reader may 
recall that we defined in a previous chapter a quantity 77, which 
is the number of fission neutrons produced for each thermal 
neutron absorbed in uranium regardless of the details of the 
process. If, therefore, we multiply the number of thermal neu- 
trons absorbed in uranium, Ncpf, by 77, we have the number of 
new high speed neutrons generated by the original N high speed 
neutrons in the course of their lives. If Nepfr; is greater than N, 
we have a chain reaction and the number of neutrons is con- 
tinually increasing. Evidently the product epf?; = k^, the multi- 
plication factor already defined in Chapter IV. 

8.13. Note that no mention has been made of neutrons 
escaping from the pile. Such mention has been deliberately 
avoided since the value of k^ as defined above applies to an 
infinite lattice. From the known values of k^ and the fact that 
these piles do operate, one finds that the percentage of neutrons 
escaping cannot be very great. As we saw in Chapter II, the 
escape of neutrons becomes relatively less important as the size 
of the pile increases. If it is necessary to introduce in the pile a 
large amount of auxiliary material such as cooling-system pipes, 
it is necessary to build a somewhat larger pile to counteract the 
increase in absorption. 

8.14. To sum up, a pile operates by reducing high-energy 
neutrons to thermal energies by the use of a moderator-lattice 
arrangement, then allowing the thermal-energy neutrons to be 
absorbed by uranium, causing fission which regenerates further 
high-energy neutrons. The regeneration of neutrons is aided 
slightly by the fast neutron effect; it is impeded by resonance 
absorption during the process of energy reduction, by absorption 
in graphite and other materials, and by neutron escape. 


8.15. Even at the high power level used in the Hanford piles, 
only a few grams of U-238 and of U-235 are used up per day per 

Plutonium Problem to June 1945 135 

million grams of uranium present. Nevertheless the effects of 
these changes are very important. As the U-235 is becoming 
depleted, the concentration of plutonium is increasing. For- 
tunately, plutonium itself is fissionable by thermal neutrons and 
so tends to counterbalance the decrease of U-235 as far as main- 
taining the chain reaction is concerned. However, other fission 
products are being produced also. These consist typically of 
unstable and relatively unfamiliar nuclei so that it was originally 
impossible to predict how great an undesirable effect they would 
have on the multiplication constant. Such deleterious effects are 
called poisoning. 


8.16. There are two main parts of the plutonium production 
process at Hanford : actual production in the pile, and separation 
of the plutonium from the uranium slugs in which it is formed. 
We turn now to a discussion of the second part, the separation 

8.17. The uranium slugs containing plutonium also contain 
other elements resulting from the fission of U-235. When a U-235 
nucleus undergoes fission, it emits one or more neutrons and 
splits into two fragments of comparable size and of total mass 
235 or less. Apparently fission into precisely equal masses rarely 
occurs, the most abundant fragments being a fragment of mass 
number between 134 and 144 and a fragment of mass number 
between 100 and 90. Thus there are two groups of fission prod- 
ucts: a heavy group with mass numbers extending approxi- 
mately from 127 to 154, and a light group from approximately 
1 15 to 83. These fission products are in the main unstable isotopes 
of the thirty or so known elements in these general ranges of mass 
number. Typically they decay by successive beta emissions 
accompanied by gamma radiation finally to form known stable 
nuclei. The half-lives of the various intermediate nuclei range 
from fractions of a second to a year or more; several of the 
important species have half-lives of the order of a month or so. 

136 Plutonium Problem to June 1945 

About twenty different elements are present in significant con- 
centration. The most abundant of these comprises slightly less 
than 10 per cent of the aggregate. 

8.18. In addition to radioactive fission products, U-239 and 
Np-239 (intermediate products in the formation of plutonium) 
are present in the pile and are radioactive. The concentrations 
of all these products begin to build up at the moment the pile 
starts operating. Eventually the rate of radioactive decay equals 
the rate of formation so that the concentrations become constant. 
For example, the number of atoms of U-239 produced per second 
is constant for a pile operating at a fixed power level. According 
to the laws of radioactive disintegration, the number of U-239 
atoms disappearing per second is proportional to the number of 
such atoms present and is thus increasing during the first few 
minutes or hours after the pile is put into operation. Consequently 
there soon will be practically as many nuclei disintegrating each 
second as are formed each second. Equilibrium concentrations 
for other nuclei will be approached in similar manner, the 
equilibrium concentration being proportional to the rate of 
formation of the nucleus and to its half-life. Products which are 
stable or of extremely long half-life (e.g., plutonium) will steadily 
increase in concentration for a considerable time. When the pile 
is stopped, the radioactivity of course continues, but at a con- 
tinually diminishing absolute rate. Isotopes of very short half- 
life may "drop out of sight" in a few minutes or hours; others of 
longer half-life keep appreciably active for days or months. 
Thus at any time the concentrations of the various products in a 
recently stopped pile depend on what the power level was, on 
how long the pile ran, and on how long it has been shut down. 
Of course, the longer the pile has run, the larger is the concentra- 
tion of plutonium and (unfortunately) the larger is the concentra- 
tion of long-lived fission products. The longer the "cooling" 
period, i.e., the period between removal of material from the 
pile and chemical treatment, the lower is the radiation intensity 
from the fission products. A compromise must be made between 
such considerations as the desire for a long running and cooling 

Plutonium Problem to June 1945 137 

time on the one hand and the desire for early extraction of the 
plutonium on the other hand. 

8.19. Tables can be prepared showing the chemical concentra- 
tions of plutonium and the various fission products as functions 
of power level, length of operation, and length of cooling period. 
The half-life of the U-239 is so short that its concentration be- 
comes negligible soon after the pile shuts down. The neptunium 
becomes converted fairly rapidly to plutonium. Of course, the 
total weight of fission products, stable and unstable, remains 
practically constant after the pile is stopped. For the Clinton and 
Hanford operating conditions the maximum plutonium con- 
centration attained is so small as to add materially to the diffi- 
culty of chemical separation. 


8.20. The problem then is to make a chemical separation at 
the daily rate of, say, several grams of plutonium from several 
thousand grams of uranium contaminated with large amounts of 
dangerously radioactive fission products comprising twenty 
different elements. The problem is especially difficult as the 
plutonium purity requirements are very high indeed. 

8.21. Four types of method for chemical separation were 
examined: volatility, absorption, solvent extraction, and pre- 
cipitation. The work on absorption and solvent extraction 
methods has been extensive and such methods may be increas- 
ingly used in the main process or in waste recovery, but the Han- 
ford Plant was designed for a precipitation process. 

8.22. * The phenomena of co-precipitation, i.e., the precipita- 
tion of small concentrations of one element along with a "carrier" 
precipitate of some other element, had been commonly used in 
radioactive chemistry, and was adopted for plutonium separation. 
The early work on plutonium chemistry, confined as it was to 
minute amounts of the element, made great use of precipitation 

* Paragraphs 8.22-8.26 are quoted or paraphrased from a general report 
of the Metallurgical Laboratory prepared in the spring of 1945. 

138 Plutonium Problem to June 1945 

reactions from which solubility properties could be deduced. 
It was therefore natural that precipitation methods of separation 
were the most advanced at the time when the plant design was 
started. It was felt that, should the several steps in the separations 
process have to be developed partly by the empirical approach, 
there would be less risk in the scale-up of a precipitation process 
than, for example, of one involving solid-phase reactions. In 
addition, the precipitation processes then in mind could be 
broken into a sequence of repeated operations (called cycles), 
thereby limiting the number of different equipment pieces 
requiring design and allowing considerable process change 
without equipment change. Thus, while the basic plant design 
was made with one method in mind, the final choice of a different 
method led to no embarrassments. 

8.23. Most of the precipitation processes which have received 
serious consideration made use of an alternation between the 
(IV) and (VI) oxidation states of plutonium. Such processes 
involve a precipitation of plutonium (IV) with a certain com- 
pound as a carrier, then dissolution of the precipitate, oxidation 
of the plutonium to the (VI) state, and reprecipitation of the 
carrier compound while the plutonium (VI) remains in solution. 
Fission products which are not carried by these compounds 
remain in solution when plutonium (IV) is precipitated. The 
fission products which carry are removed from the plutonium 
when it is in the (VI) state. Successive oxidation-reduction 
cycles are carried out until the desired decontamination is 
achieved. The process of elimination of the fission products is 
called decontamination and the degree of elimination is tested 
by measuring the change in radioactivity of the material. 


8.24. It is possible to combine or couple the various types of 
process. Some advantages may be gained in this way since one 
type of process may supplement another. For example, a process 
which gives good decontamination might be combined advan- 

MAN MA n AN UiS'l KiCl 


"rf^fc^l** -% 

^^^ : j**** 
*#* _ 

Above: Administration Building for the Manhattan Engineer District at Oak Ridge. 
Tenn. Below: Hanford, near Pasco, Wash., which at one time housed thousands of 
workers who built the plants at the Hanford Engineer Works. Now a ghost town. 

Site diagram of the Hanford Engineer Works near Pasco, Wash. 


-r V 






Initial test of the atomic bomb in New Mexico on July 16, from a distance of 6 miles. 
Above: The start of the explosion. This small cloud later rose to a height of 40,000 feet. 
Center: Multi-colored cloud from the explosion. Black areas were brighter than the 
sun itself, according to observers. Below. A later stage of the development of the cloud. 

Plutonium Problem to June 1945 139 

tageously with one which, while inefficient for decontamination, 
would be very efficient for separation from uranium. 

8.25. At the time when it became necessary to decide on the 
process to serve as the basis for the design of the Hanford plant 
(June 1943), the choice, for reasons given above, was limited to 
precipitation processes and clearly lay between two such processes. 
However, the process as finally chosen actually represented a 
combination of the two. 

8.26. The success of the separation process at Hanford has 
exceeded all expectations. The high yields and decontamination 
factors and the relative ease of operation have amply demon- 
strated the wisdom of its choice as a process. This choice was 
based on a knowledge of plutonium chemistry which had been 
gleaned from less than a milligram of plutonium. Further devel- 
opments may make the present Hanford process obsolete, but 
the principal goal, which was to have a workable and efficient 
process for use as soon as the Hanford piles were delivering pluton- 
ium, has been attained. 


8.27. The Argonne Laboratory was constructed early in 1943 
outside Chicago. The site, originally intended for a pilot plant, 
was later considered to be too near the city and was used for 
reconstructing the so-called West Stands pile which was orig- 
inally built on the University of Chicago grounds and which was 
certainly innocuous. Under the direction of E. Fermi and his 
colleagues, H. L. Anderson, W. H. Zinn, G. Weil, and others, 
this pile has served as a prototype unit for studies of thermal 
stability, controls, instruments, and shielding, and as a neutron 
source for materials testing and neutron-physics studies. Further- 
more, it has proved valuable as a training school for plant oper- 
ators. More recently a heavy- water pile (see below) has been 
constructed there. 

8.28. The first Argonne pile, a graphite-uranium pile, need 
not be described in detail. The materials and lattice structure 
are nearly identical to those which were used for the original 

140 Plutonium Problem to June 1945 

West Stands pile. The pile is a cube; it is surrounded by a shield 
and has controls and safety devices somewhat similar to those 
used later at Clinton. It has no cooling system and is normally 
run at a power level of only a few kilowatts. It has occasionally 
been run at high-power levels for very brief periods. Considering 
that it is merely a reconstruction of the first chain-reacting unit 
ever built, it is amazing that it has continued in operation for 
more than two years without developing any major troubles. 

8.29. One of the most valuable uses of the Argonne pile has 
been the measurement of neutron-absorption cross sections of a 
great variety of elements which might be used in piles as struc- 
tural members, etc., or which might be present in pile materials 
as impurities. These measurements are made by observing the 
change in the controls necessary to make kff equal to 1 .00 when 
a known amount of the substance under study is inserted at a 
definite position in the pile. The results obtained were usually 
expressed in terms of "danger coefficients." 

8.30. An opening at the top of the pile lets out a very uniform 
beam of thermal neutrons that can be used for exponential-pile 
experiments, for direct measurements of absorption cross sections, 
for Wilson cloud chamber studies, etc. 

8.31. An interesting phenomenon occurring at the top of the 
pile is the production of a beam or flow of "cold" neutrons. If a 
sufficient amount of graphite is interposed between the upper 
surface of the pile and an observation point a few yards above, 
the neutron energy distribution is found to correspond to a 
temperature much lower than that of the graphite. This is pre- 
sumed to be the result of a preferential transmission by the 
crystalline graphite of the slowest ("coldest") neutrons, whose 
quantum-mechanical wave-length is great compared to the 
distance between successive planes in the graphite crystals. 

8.32. More recently a pile using heavy water as moderator 
was constructed in the Argonne Laboratory. The very high 
intensity beam of neutrons produced by this pile has been found 
well-suited to the study of "neutron optics," e.g., reflection and 
refraction of neutron beams as by graphite. 

Plutonium Problem to June 1945 141 

8.33. A constant objective of the Argonne Laboratory has been 
a better understanding of nuclear processes in uranium, neptun- 
ium, and plutonium. Repeated experiments have been made to 
improve the accuracy of constants such as thermal-fission cross 
sections of U-235, U-238, and Pu-239, probabilities of non-fission 
neutron absorption by each of these nuclei, and number of neu- 
trons emitted per fission. 


8.34. In Chapter VI we mentioned plans for a "pilot" plant 
for production of plutonium to be built at the Clinton site in 
Tennessee. By January 1943, the plans for this project were well 
along; construction was started soon afterward. M. D. Whitaker 
was appointed director of the Clinton Laboratories. The pilot- 
plant plans were made cooperatively by du Pont and the Metal- 
lurgical Laboratory; construction was carried out by du Pont; 
plant operation was maintained by the University of Chicago 
as part of the Metallurgical Project. 

8.35. The main purposes of the Clinton plant were to produce 
some plutonium and to serve as a pilot plant for chemical separa- 
tion. As regards research, the emphasis at Clinton was on chem- 
istry and on the biological effects of radiations. A large laboratory 
was provided for chemical analysis, for research on purification 
methods, for fission-product studies, for development of inter- 
mediate-scale extraction and decontamination processes, etc. 
Later a "hot laboratory," i.e., a laboratory for remotely-con- 
trolled work on highly radioactive material, was provided. There 
is also an instrument shop and laboratory that has been used very 
actively. There are facilities for both clinical and experimental 
work of the health division, which has been very active. There is 
a small physics laboratory in which some important work was 
done using higher neutron intensities than were available at the 
Argonne Laboratory. The principal installations constructed at 
the Clinton Laboratory site were the pile and the separation 
plant; these are briefly described below. 

142 Plutonium Problem to June 1945 


8.36. In any steadily operating pile the effective multiplication 
factor k must be kept at 1, whatever the power level. The best 
k^ that had been observed in a uranium-graphite lattice could 
not be achieved in a practical pile because of neutron leakage, 
cooling system, cylindrical channels for the uranium, protective 
coating on the uranium, and other minor factors. Granted air- 
cooling and a maximum safe temperature for the surface of the 
uranium, a size of pile had to be chosen that could produce 
1 ,000 kw. The effective k would go down with rising temperature 
but not sufficiently to be a determining factor. Though a sphere 
was the ideal shape, practical considerations recommended a 
rectangular block. 

8.37. The Clinton pile consists of a cube of graphite containing 
horizontal channels filled with uranium. The uranium is in the 
form of metal cylinders protected by gas-tight casings of alumi- 
num. The uranium cylinders or slugs may be slid into the chan- 
nels in the graphite; space is left to permit cooling air to flow past, 
and to permit pushing the slugs out at the back of the pile when 
they are ready for processing. Besides the channels for slugs there 
are various other holes through the pile for control rods, instru- 
ments, etc. 

8.38. The Clinton pile was considerably larger than the first 
pile at Chicago (see Chapter VI). More important than the 
increased size of the Clinton pile were its cooling system, heavier 
shields, and means for changing the slugs. The production goal 
of the Clinton plant was set at a figure which meant that the pile 
should operate at a power level of 1,000 kw. 

8.39. The instrumentation and controls are identical in princi- 
ple to those of the first pile. Neutron intensity in the pile is 
measured by a BFs ionization chamber and is controlled by boron 
steel rods that can be moved in and out of the pile, thereby vary- 
ing the fraction of neutrons available to produce fission. 

8.40. In spite of an impressive array of instruments and safety 
devices, the most striking feature of the pile is the simplicity of 

Plutonium Problem to June 1945 143 

operation. Most of the time the operators have nothing to do 
except record the readings of various instruments. 


8.41. Here, as at Hanford, the plutonium processes have to 
be carried out by remote control and behind thick shields. The 
separation equipment is housed in a series of adjacent cells having 
heavy concrete walls. These cells form a continuous structure 
(canyon) which is about 100 feet long and is two- thirds buried in 
the ground. Adjacent to this canyon are the control rooms, 
analytical laboratories, and a laboratory for further purification 
of the plutonium after it has been decontaminated to the point 
of comparative safety. 

8.42. Uranium slugs that have been exposed in the pile are 
transferred under water to the first of these cells and are then 
dissolved. Subsequent operations are performed by pumping 
solutions or slurries from one tank or centrifuge to another. 


8.43. The Clinton pile started operating on November 4, 1943, 
and within a few days was brought up to a power level of 500 kw 
at a maximum slug surface temperature of 110 C. Improvements 
in the air circulation and an elevation of the maximum uranium 
surface temperature to 150 C. brought the power level up to 
about 800 kw, where it was maintained until the spring of 1944. 
Starting at that time, a change was made in the distribution of 
uranium, the change being designed to level out the power 
distribution in the pile by reducing the amount of metal near the 
center relative to that further out and thereby to increase the 
average power level without anywhere attaining too high a tem- 
perature. At the same time improvements were realized in the 
sealing of the slug jackets, making it possible to operate the pile 
at higher temperature. As a result, a power level of 1,800 kw was 
attained in May 1944; this was further increased after the 
installation of better fans in June 1944. 

144 Plutonium Problem to June 1945 

8.44. Thus the pile performance of June 1944 considerably 
exceeded expectations. In ease of control, steadiness of operation, 
and absence of dangerous radiation, the pile has been most 
satisfactory. There have been very few failures attributable to 
mistakes in design or construction. 

8.45. The pile itself was simple both in principle and in 
practice. Not so the plutonium-separation plant. The step from 
the first chain-reacting pile to the Clinton pile was reasonably 
predictable; but a much greater and more uncertain step was 
required in the case of the separation process, for the Clinton 
separation plant was designed on the basis of experiments using 
only microgram amounts of plutonium. 

8.46. Nevertheless, the separation process worked! The first 
batch of slugs from the pile entered the separation plant on 
December 20, 1943. By the end of January 1944, metal from the 
pile was going to the separation plant at the rate of % ton per 
day. By February 1 , 1 944, 1 90 mg of plutonium had been delivered 
and by March 1, 1944, several grams had been delivered. Further- 
more, the efficiency of recovery at the very start was about 50 per 
cent, and by June 1 944 it was between 80 and 90 per cent. 

8.47. During this whole period there was a large group of 
chemists at Clinton working on improving the process and 
developing it for Hanford. The Hanford problem differed from 
that at Clinton in that much higher concentrations of plutonium 
were expected. Furthermore, though the chemists were to be 
congratulated on the success of the Clinton plant, the process 
was complicated and expensive. Any improvements in yield or 
decontamination or in general simplification were very much to 
be sought. 

8.48. Besides the proving of the pile and the separation plant 
and the production of several grams of plutonium for experimental 
use at Chicago, Clinton, and elsewhere, the Clinton Laboratories 
have been invaluable as a training and testing center for Hanford, 
for medical experiments, pile studies, purification studies, and 
physical and chemical studies of plutonium and fission products. 

8.49. As typical of the kind of problems tackled there and at 

Plutonium Problem to June 1945 145 

Chicago, the following problems listed in a single routine report 
for May 1944 are pertinent: 

Problems Closed Out during May 1944: Search for New Oxidizing 
Agent, Effect of Radiation on Water and Aqueous Solutions, 
Solubility of Plutonium Peroxide, Plutonium Compounds Suit- 
able for Shipment, Fission Product Distribution in Plant Process 
Solutions, Preliminary Process Design for Adsorption Extraction, 
Adsorption Semi- Works Assistance, Completion of Adsorption 
Process Design. 

New Problems Assigned during May 1944: New Product Analysis 
Method, Effect of Radiation on Graphite, Improvement in 
Yield, New Pile Explorations, Waste Uranium Recovery, 
Monitoring Stack Gases, Disposal of Active Waste Solutions, 
Spray Cooling of X Pile, Assay Training Program, Standardiza- 
tion of Assay Methods, Development of Assay Methods, Shielded 
Apparatus for Process Control Assays, Cloud Chamber Experi- 
ment, Alpha Particles from U-235, Radial Product Distribution, 
Diffraction of Neutrons. 


8.50. It is beyond the scope of this report to give any account 
of the construction of the Hanford Engineer Works, but it is to 
be hoped that the full story of this extraordinary enterprise and 
the companion one, the Clinton Engineer Works, will be pub- 
lished at some time in the future. The Hanford site was examined 
by representatives of General Groves and of du Pont at the end 
of 1 942, and use of the site was approved by General Groves after 
he had inspected it personally. It was on the west side of the 
Columbia River in central Washington north of Pasco. In the 
early months of 1943 a 200-square-mile tract in this region was 
acquired by the government (by lease or purchase) through the 
Real Estate Division of the Office of the Chief of Engineers. 
Eventually an area of nearly a thousand square miles was brought 
under government control. At the time of acquisition of the land 
there were a few farms and two small villages, Hanford and 
Richland, on the site, which was otherwise sage-brush plains 

146 Plutonium Problem to June 1945 

and barren hills. On the 6th of April, 1943, ground was broken 
for the Hanford construction camp. At the peak of activity in 
1944, this camp was a city of 60,000 inhabitants, the fourth largest 
city in the state. Now, however, the camp is practically deserted 
as the operating crew is housed at Richland. 

8.51. Work was begun on the first of the Hanford production 
piles on June 7, 1943, and operation of the first pile began in 
September 1944. The site was originally laid out for five piles, 
but the construction of only three has been undertaken. Besides 
the piles, there are, of course, plutonium separation plants, 
pumping stations and water-treatment plants. There is also a 
low-power chain-reacting pile for material testing. Not only are 
the piles themselves widely spaced for safety several miles apart 
but the separation plants are well away from the piles and 
from each other. All three piles were in operation by the summer 
of 1945. 


8.52. No one who lived through the period of design and con- 
struction of the Hanford plant is likely to forget the "canning" 
problem, i.e., the problem of sealing the uranium slugs in pro- 
tective metal jackets. On periodic visits to Chicago the writer 
could roughly estimate the state of the canning problem by the 
atmosphere of gloom or joy to be found around the laboratory. 
It was definitely not a simple matter to find a sheath that would 
protect uranium from water corrosion, would keep fission products 
out of the water, would transmit heat from the uranium to the 
water, and would not absorb too many neutrons. Yet the failure 
of a single can might conceivably require shut-down of an entire 
operating pile. 

8.53. Attempts to meet the stringent requirements involved 
experimental work on electroplating processes, hot-dipping 
processes, cementation-coating processes, corrosion-resistant alloys 
of uranium, and mechanical jacketing or canning processes. 
Mechanical jackets or cans of thin aluminum were feasible from 
the nuclear-physics point of view and were chosen early as the 

Plutonium Problem to June 1945 147 

most likely solution of the problem. But the problem of getting 
a uniform, heat-conducting bond between the uranium and the 
surrounding aluminum, and the problem of effecting a gas-tight 
closure for the can both proved very troublesome. Development 
of alternative methods had to be carried along up to the last 
minute, and even up to a few weeks before it was time to load 
the uranium slugs into the pile there was no certainty that any 
of the processes under development would be satisfactory. A 
final minor but apparently important modification in the pre- 
ferred canning process was adopted in October 1944, after the 
first pile had begun experimental operation. By the summer of 
1945, there had been no can failure reported. 


8.54. During the fall of 1944 and the early months of 1945 
the second and third Hanford piles were finished and put into 
operation, as were the additional chemical separation plants. 
There were, of course, some difficulties; however, none of the 
fears expressed as to canning failure, film formation in the water 
tubes, or radiation effects in the chemical processes, have turned 
out to be justified. As of early summer 1945 the piles are operat- 
ing at designed power, producing plutonium, and heating the 
Columbia River.* The chemical plants are separating the plu- 
tonium from the uranium and from the fission products with 
better efficiency than had been anticipated. The finished product 
is being delivered. How it can be used is the subject of Chapter 


8.55. In previous chapters there have been references to the 
advantages of heavy water as a moderator. It is more effective 
than graphite in slowing down neutrons and it has a smaller 
neutron absorption than graphite. It is therefore possible to build 

* The actual rise in temperature is so tiny that no effect on fish life could 
be expected. To make doubly sure, this expectation was confirmed by an 
elaborate series of experiments. 

148 Plutonium Problem to June 1945 

a chain-reacting unit with uranium and heavy water and thereby 
to attain a considerably higher multiplication factor, k, and a 
smaller size than is possible with graphite. But one must have 
the heavy water. 

8.56. In the spring of 1943 the Metallurgical Laboratory 
decided to increase the emphasis on experiments and calculations 
aimed at a heavy- water pile. To this end a committee was set up 
under E. Wigner, a group under H. C. Vernon was transferred 
from Columbia to Chicago, and H. D. Smyth, who had just 
become associate director of the Laboratory, was asked to take 
general charge. 

8.57. The first function of this group was to consider in what 
way heavy water could best be used to insure the overall success 
of the Metallurgical Project, taking account of the limited produc- 
tion schedule for heavy water that had been already authorized. 

8.58. It became apparent that the production schedule was 
so low that it would take two years to produce enough heavy 
water to "moderate" a fair-sized pile for plutonium production. 
On the other hand, there might be enough heavy water to moder- 
ate a small "laboratory" pile, which could furnish information 
that might be valuable. In any event, during the summer of 1943 
so great were the uncertainties as to the length of the war and 
as to the success of the other parts of the DSM project that a 
complete study of the possibilities of heavy-water piles seemed 
desirable. Either the heavy-water production schedule might be 
stepped up or the smaller, experimental pile might be built. An 
intensive study of the matter was made during the summer of 
1943 but in November it was decided to curtail the program 
and construction was limited to a 250-kw pile located at the 
Argonne site. 


8.59. Perhaps the most striking aspect of the uranium and 
heavy-water pile at the Argonne is its small size. Even with its 
surrounding shield of concrete it is relatively small compared 
to the uranium-graphite piles. 

Plutonium Problem to June 1945 149 

8.60. By May 15, 1944, the Argonne uranium and heavy-water 
pile was ready for test. With the uranium slugs in place, it was 
found that the chain reaction in the pile became self sustaining 
when only three fifths of the heavy water had been added. The 
reactivity of the pile was so far above expectations that it would 
have been beyond the capacity of the control rods to handle if 
the remainder of the heavy water had been added. To meet this 
unusual and pleasant situation some of the uranium was removed 
and extra control rods were added. 

8.61. With these modifications it was possible to fill the tank 
to the level planned. By July 4, 1944, W. H. Zinn reported that 
the pile was running satisfactorily at 190 kw, and by August 8, 
1 944, he reported that it was operating at 300 kw. 

8.62. In general the characteristics of this pile differed slightly 
from those of comparable graphite piles. This pile takes several 
hours to reach equilibrium. It shows small (less than 1 per cent) 
but sudden fluctuations in power level, probably caused by 
bubbles in the water. It cannot be shut down as completely or 
as rapidly as the graphite pile because of the tendency of delayed 
gamma rays to produce (from the heavy water) additional 
neutrons. As anticipated, the neutron density at the center is 
high. The shields, controls, heat exchanger, etc., have operated 


8.63. The major objective of the health group was in a sense a 
negative one, to insure that no one concerned suffered serious 
injury from the peculiar hazards of the enterprise. Medical case 
histories of persons suffering serious injury or death resulting 
from radiation were emphatically not wanted. The success of the 
health division in meeting these problems was remarkable. Even 
in the research group where control is most difficult, cases show- 
ing even temporary bad effects were extremely rare. Factors of 
safety used in plant design and operation are so great that the 
hazards of the home and the family car are far greater for the 
personnel than any arising from the plants. 

150 Plutonium Problem to June 1945 

8.64. To achieve its objective the health group worked along 
three major lines: 

(1) Adoption of pre-employment physical examinations and 
frequent re-examinations, particularly of those exposed to 

(2) Setting of tolerance standards for radiation doses and 
development of instruments measuring exposure of personnel; 
giving advice on shielding, etc. ; continually measuring radiation 
intensities at various locations in the plants; measuring con- 
tamination of clothes, laboratory desks, waste water, the atmos- 
phere, etc. 

(3) Carrying out research on the effects of direct exposure of 
persons and animals to various types of radiation, and on the 
effects of ingestion and inhalation of the various radioactive or 
toxic materials such as fission products, plutonium and uranium. 


8.65. The white blood-corpuscle count was used as the princi- 
pal criterion as to whether a person suffered from overexposure 
to radiation. A number of cases of abnormally low counts were 
observed and correlated with the degree of overexposure. Indi- 
viduals appreciably affected were shifted to other jobs or given 
brief vacations; none has shown permanent ill effects. 

8.66. At the same time it was recognized that the white blood- 
corpuscle count is not an entirely reliable criterion. Some work 
on animals indicated that serious damage might occur before 
the blood count gave any indication of danger. Accordingly, 
more elaborate blood tests were made on selected individuals 
and on experimental animals in the hope of finding a test that 
would give an earlier warning of impending injury. 


8.67. The Health Division had principal responsibility for the 
development of pocket meters for indicating the extent of ex- 
posure of persons. The first of these instruments was a simple 

Plutonium Problem to June 1945 151 

electroscope about the size and shape of a fountain pen. Such 
instruments were electrostatically charged at the start of each 
day and were read at the end of the day. The degree to which 
they became discharged indicated the total amount of ionizing 
radiation to which they had been exposed. Unfortunately they 
were none too rugged and reliable, but the error of reading was 
nearly always in the right direction i.e., in the direction of over- 
stating the exposure. At an early date the practice was estab- 
lished of issuing two of these pocket meters to everyone entering 
a dangerous area. A record was kept of the readings at the time 
of issuance and also when the meters were turned in. The meters 
themselves were continually although gradually improved. The 
Health Division later introduced "film badges," small pieces of 
film worn in the identification badge, the films being periodically 
developed and examined for radiation blackening. These instru- 
ments for individuals such as the pocket meter and film badge 
were extra and probably unnecessary precautions. In permanent 
installations the shielding alone normally affords complete safety. 
Its effect is under frequent survey by either permanently installed 
or portable instruments. 

8.68. The Health Division cooperated with the Physics Division 
in the development and use of various other instruments. There 
was "Sneezy" for measuring the concentration of radioactive 
dust in the air and "Pluto" for measuring a-emitting contamina- 
tion (usually plutonium) of laboratory desks and equipment. 
Counters were used to check the contamination of laboratory 
coats before and after the coats were laundered. At the exit gates 
of certain laboratories concealed counters sounded an alarm 
when someone passed whose clothing, skin or hair was con- 
taminated. In addition, routine inspections of laboratory areas 
were made. 

8.69. One of the studies made involved meteorology. It be- 
came essential to know whether the stack gases (at Clinton and 
at Hanford) would be likely to spread radioactive fission products 
in dangerous concentrations. Since the behavior of these gases 
is very dependent on the weather, studies were made at both 

152 Plutonium Problem to June 1945 

sites over a period of many months, and satisfactory stack opera- 
tion was specified. 


8.70. Since both the scale and the variety of the radiation 
hazards in this enterprise were unprecedented, all reasonable 
precautions were taken; but no sure means were at hand for 
determining the adequacy of the precautions. It was essential to 
supplement previous knowledge as completely as possible. For 
this purpose, an extensive program of animal experimentation 
was carried out along three main lines: (1) exposure to neutron, 
alpha, beta and gamma radiation; (2) ingestion of uranium, 
plutonium and fission products; (3) inhalation of uranium, 
plutonium and fission products. Under the general direction of 
Dr. Stone these experiments were carried out at Chicago, Clinton 
and the University of California principally by Dr. Cole and Dr. 
Hamilton. Extensive and valuable results were obtained. 


8.71. Both space and security restrictions prevent a detailed 
report on the work of the laboratories and plants concerned with 
plutonium production. 

8.72. Two types of neutron absorption are fundamental to the 
operation of the plant: one, neutron absorption in U-235 result- 
ing in fission, maintains the chain reaction as a source of neutrons; 
the other, neutron absorption in U-238 leads to the formation of 
plutonium, the desired product. 

8.73. The course of a nuclear chain reaction in a graphite- 
moderated heterogeneous pile can be described by following a 
single generation of neutrons. The original fast neutrons are 
slightly increased in number by fast fission, reduced by resonance 
absorption in U-238 and further reduced by absorption at thermal 
energies in graphite and other materials and by escape; the 
remaining neutrons, which have been slowed in the graphite, 
cause fission in U-235, producing a new generation of fast 
neutrons similar to the previous generation. 

Plutonium Problem to June 7945 153 

8.74. The product, plutonium, must be separated by chemical 
processes from a comparable quantity of fission products and a 
much larger quantity of uranium. Of several possible separation 
processes the one chosen consists of a series of reactions including 
precipitating with carriers, dissolving, oxidizing and reducing. 

8.75. The chain reaction was studied at low power at the 
Argonne Laboratory beginning early in 1943. Both chain reaction 
and chemical separation processes were investigated at the 
Clinton Laboratories beginning in November 1943, and an 
appreciable amount of plutonium was produced there. 

8.76. Construction of the main production plant at Hanford, 
Washington, was begun in 1943 and the first large pile went into 
operation in September 1 944. The entire plant was in operation 
by the summer of 1945 with all chain-reacting piles and chemical- 
separation plants performing better than had been anticipated. 

8.77. Extensive studies were made on the use of heavy water 
as a moderator and an experimental pile containing heavy water 
was built at the Argonne Laboratory. Plans for a production 
plant using heavy water were given up. 

8.78. The Health Division was active along three main lines: 
(1) medical examination of personnel; (2) advice on radiation 
hazards and constant check on working conditions; (3) research 
on the effects of radiation. The careful planning and exhaustive 
research work of this division have resulted in an outstanding 
health record at Hanford and elsewhere in the project. 



9.1. The possibility of producing an atomic bomb of U-235 
was recognized before plutonium was discovered. Because it was 
appreciated at an early date that the separation of the uranium 
isotopes would be a direct and major step toward making such a 
bomb, methods of separating uranium isotopes have been under 
scrutiny for at least six years. Nor was attention confined to 
uranium since it was realized that the separation of deuterium 
was also of great importance. In the present chapter the general 
problems of isotope separation will be discussed; later chapters 
will take up the specific application of various processes. 


9.2. By definition, the isotopes of an element differ in mass but 
not in chemical properties. More precisely, although the nuclear 
masses and structures differ, the nuclear charges are identical 
and therefore the external electronic structures are practically 
identical. For most practical purposes, therefore, the isotopes of 
an element are separable only by processes depending on the 
nuclear mass. 

9.3. It is well known that the molecules of a gas or liquid are 
in continual motion and that their average kinetic energy depends 
only on the temperature, not on the chemical properties of the 
molecules. Thus in a gas made up of a mixture of two isotopes 
the average kinetic energy of the light molecules and of the heavy 
ones is the same. Since the kinetic energy of a molecule is J^ mv 2 , 
where m is the mass and v the speed of the molecule, it is appar- 
ent that on the average the speed of a lighter molecule must be 


The Separation of Isotopes 155 

greater than that of a heavier molecule. Therefore, at least in 
principle, any process depending on the average speed of mole- 
cules can be used to separate isotopes. Unfortunately, the average 
speed is inversely proportional to the square root of the mass so 
that the difference is very small for the gaseous compounds of 
the uranium isotopes. Also, although the average speeds differ, the 
ranges of speed show considerable overlap. In the case of the gas 
uranium hexafluoride, for example, over 49 per cent of the light 
molecules have speeds as low as those of 50 per cent of the heavy 

9.4. Obviously there is no feasible way of applying mechanical 
forces directly to molecules individually; they cannot be poked 
with a stick or pulled with a string. But they are subject to gravi- 
tational fields and, if ionized, may be affected by electric and 
magnetic fields. Gravitational forces are, of course, proportional 
to the mass. In a very high vacuum U-235 atoms and U-238 atoms 
would fall with the same acceleration, but just as a feather and 
a stone fall at very different rates in air where there are frictional 
forces resisting motion, there may be conditions under which a 
combination of gravitational and opposing intermolecular forces 
will tend to move heavy atoms differently from light ones. 
Electric and magnetic fields are more easily controlled than 
gravitational fields or "pseudogravitational" fields (i.e., centrif- 
ugal-force fields) and are very effective in separating ions of 
differing masses. 

9.5. Besides gravitational or electromagnetic forces, there are, 
of course, interatomic and intermolecular forces. These forces 
govern the interaction of molecules and thus affect the rates of 
chemical reactions, evaporation processes, etc. In general, such 
forces will depend on the outer electrons of the molecules and not 
on the nuclear masses. However, whenever the forces between 
separated atoms or molecules lead to the formation of new 
molecules, a mass effect (usually very small) does appear. In 
accordance with quantum-mechanical laws, the energy levels 
of the molecules are slightly altered, and differently for each 
isotope. Such effects do slightly alter the behavior of two isotopes 

156 The Separation of Isotopes 

in certain chemical reactions, as we shall see, although the dif- 
ference in behavior is far smaller than the familiar differences of 
chemical behavior between one element and another. 

9.6. These, then, are the principal factors that may have to be 
considered in devising a separation process: equality of average 
thermal kinetic energy of molecules at a given temperature, 
gravitational or centrifugal effects proportional to the molecular 
masses, electric or magnetic forces affecting ionized molecules, 
and interatomic or intermolecular forces. In some isotope- 
separation processes only one of these effects is involved and the 
overall rate of separation can be predicted. In other isotope- 
separation processes a number of these effects occur simultane- 
ously so that prediction becomes difficult. 


9.7. Before discussing particular processes suitable for isotope 
separation, we should know what is wanted. The major criteria 
to be used in judging an isotope-separation process are as follows. 


9.8. The separation factor, sometimes known as the enrich- 
ment or fractionating factor of a process, is the ratio of the relative 
concentration of the desired isotope after processing to its relative 
concentration before processing. Defined more precisely: if, before 
the processing, the numbers of atoms of the isotopes of mass 
number mi and m 2 are ni and n 2 respectively (per gram of the 
isotope mixture) and if, after the processing, the corresponding 
numbers are n^ and n' 2 , then the separation factor is: 

ni/n a 

This definition may be applied to one stage of a separation plant 
or to an entire plant consisting of many stages. We are usually 
interested either in the "single stage" separation factor or in the 
"overall" separation factor of the whole process. If r is only 
slightly greater than unity, as is often the case for a single stage, 

The Separation of Isotopes 1 57 

the number r 1 is sometimes more useful than r. The quantity 
r 1 is called the enrichment factor. In natural uranium 
mi = 235, m z = 238, and ni/n 2 = K40 approximately, but 
in 90 per cent U-235, n\/n' 2 %. Consequently in a process 
producing 90 per cent U-235 from natural uranium the overall 
value of r must be about 1,260. 


9.9. In nearly every process a high separation factor means a 
low yield, a fact that calls for continual compromise. Unless 
indication is given to the contrary, we shall state yields in terms 
of U-235. Thus a separation device with a separation factor of 
2 that is, nj/n'a = Mo and a yield of one gram a day is one that, 
starting from natural uranium, produces, in one day, material 
consisting of 1 gram of U-235 mixed with 70 grams of U-238. 


9.10. The total amount of material tied up in a separation 
plant is called the "hold-up." The hold-up may be very large 
in a plant consisting of many stages. 


9.11. In a separation plant having large hold-up, a long time 
perhaps weeks or months is needed for steady operating con- 
ditions to be attained. In estimating time schedules this "start- 
up" or "equilibrium" time must be added to the time of 
construction of the plant. 


9.12. If a certain quantity of raw material is fed into a separa- 
tion plant, some of the material will be enriched, some impover- 
ished, some unchanged. Parts of each of these three fractions will 
be lost and parts recovered. The importance of highly efficient 
recovery of the enriched material is obvious. In certain processes 
the amount of unchanged material is negligible, but in others, 
notably in the electromagnetic method to be described below, it 

158 The Separation of Isotopes 

is the largest fraction and consequently the efficiency with which 
it can be recovered for recycling is very important. The impor- 
tance of recovery of impoverished material varies widely, depend- 
ing very much on the degree of impoverishment. Thus in general 
there are many different efficiencies to be considered. 


9.13. As in all parts of the uranium project, cost in time was 
more important than cost in money. Consequently a number of 
large-scale separation plants for U-235 and deuterium were built 
at costs greater than would have been required if construction 
could have been delayed for several months or years until more 
ideal processes were worked out. 



9.14. As long ago as 1896 Lord Rayleigh showed that a mixture 
of two gases of different atomic weight could be partly separated 
by allowing some of it to diffuse through a porous barrier into an 
evacuated space. Because of their higher average speed the 
molecules of the light gas diffuse through the barrier faster so 
that the gas which has passed through the barrier (i.e., the 
"diffusate") is enriched in the lighter constituent and the residual 
gas which has not passed through the barrier is impoverished 
in the lighter constituent. The gas most highly enriched in the 
lighter constituent is the so-called "instantaneous diffusate"; it 
is the part that diffuses before the impoverishment of the residue 
has become appreciable. If the diffusion process is continued 
until nearly all the gas has passed through the barrier, the average 
enrichment of the diffusate naturally diminishes. In the next 
chapter we shall consider these phenomena more fully. 
Here we shall merely point out that, on the assumption that the 
diffusion rates are inversely proportional to the square roots of 
the molecular weights the separation factor for the instantaneous 
diffusate, called the "ideal separation factor" a, is given by 

The Separation of Isotopes 159 


where MI is the molecular weight of the lighter gas and M 2 that 
of the heavier. Applying this formula to the case of uranium will 
illustrate the magnitude of the separation problem. Since uranium 
itself is not a gas, some gaseous compound of uranium must be 
used. The only one obviously suitable is uranium hexafluoride, 
UFe, which has a vapor pressure of one atmosphere at a tem- 
perature of 56 C. Since fluorine has only one isotope, the two 
important uranium hexafluorides are U 236 F 6 and U 238 F 6 ; their 
molecular weights are 349 and 352. Thus, if a small fraction of a 
quantity of uranium hexafluoride is allowed to diffuse through a 
porous barrier, the diffusate will be enriched in U 235 Fe by a factor 

= 1.0043 

which is a long way from the 1,260 required (see paragraph 9.8.). 

9.15. Such calculations might make it seem hopeless to sepa- 
rate isotopes (except, perhaps, the isotopes of hydrogen) by 
diffusion processes. Actually, however, such methods may be 
used successfully even for uranium. It was the gaseous diffusion 
method that F. W. Aston used in the first partial separation of 
isotopes (actually the isotopes of neon). Later G. Hertz and 
others, by operating multi-stage recycling diffusion units, were 
able to get practically complete separation of the neon isotopes. 
Since the multiple-stage recycling system is necessary for nearly 
all separation methods, it will be described in some detail immedi- 
ately following introductory remarks on the various methods to 
which it is pertinent. 


9.16. The separation of compounds of different boiling points, 
i.e., different vapor pressures, by distillation is a familiar indus- 
trial process. The separation of alcohol and water (between 
which the difference in boiling point is in the neighborhood of 

160 The Separation of Isotopes 

20 C.) is commonly carried out in a simple still using but a 
single evaporator and condenser. The condensed material (con- 
densate) may be collected and redistilled a number of times if 
necessary. For the separation of compounds of very nearly the 
same boiling point it would be too laborious to carry out the 
necessary number of successive evaporations and condensations 
as separate operations. Instead, a continuous separation is 
carried out in a fractionating tower. Essentially the purpose of 
a fractionating tower is to produce an upward-directed stream 
of vapor and a downward-directed stream of liquid, the two 
streams being in intimate contact and constantly exchanging 
molecules. The molecules of the fraction having the lower boiling 
point have a relatively greater tendency to get into the vapor 
stream and vice versa. Such counter-current distillation methods 
can be applied to the separation of light and heavy water, which 
differ in boiling point by 1 .4 C. 


9.17. The method of counter current flow is useful not only in 
two-phase (liquid-gas) distillation processes, but also in other 
separation processes such as those involving diffusion resulting 
from temperature variations (gradients) within one-phase systems 
or from centrifugal forces. The countercurrents may consist of 
two gases, two liquids, or one gas and one liquid. 


9.18. We have pointed out that gravitational separation of 
two isotopes might occur since the gravitational forces tending 
to move the molecules downward are proportional to the mole- 
cular weights, and the intermolecular forces tending to resist 
the downward motion depend on the electronic configuration, 
not on the molecular weights. Since the centrifuge is essentially 
a method of applying pseudogravitational forces of large magni- 
tude, it was early considered as a method for separating isotopes. 
However, the first experiments with centrifuges failed. Later 

The Separation of Isotopes 161 

development of the high speed centrifuge by J. W. Beams and 
others led to success. H. C. Urey suggested the use of tall cylin- 
drical centrifuges with countercurrent flow; such centrifuges have 
been developed successfully. 

9.19. In such a countercurrent centrifuge there is a downward 
flow of vapor in the outer part of the rotating cylinder and an 
upward flow of vapor in the central or axial region. Across the 
interface region between the two currents there is a constant 
diffusion of both types of molecules from one current to the other, 
but the radial force field of the centrifuge acts more strongly on 
the heavy molecules than on the light ones so that the concentra- 
tion of heavy ones increases in the peripheral region and decreases 
in the axial region, and vice versa for the lighter molecules. 

9.20. The great appeal of the centrifuge in the separation of 
heavy isotopes like uranium is that the separation factor depends 
on the difference between the masses of the two isotopes, not on 
the square root of the ratio of the masses as in diffusion methods. 


9.21. The kinetic theory of gases predicts the extent of the 
differences in the rates of diffusion of gases of different molecular 
weights. The possibility of accomplishing practical separation of 
isotopes by thermal diffusion was first suggested by theoretical 
studies of the details of molecular collisions and of the forces 
between molecules. Such studies made by Enskog and by Chap- 
man before 1920 suggested that if there were a temperature 
gradient in a mixed gas there would be a tendency for one type 
of molecule to concentrate in the cold region and the other in 
the hot region. This tendency depends not only on the molecular 
weights but also on the forces between the molecules. If the gas 
is a mixture of two isotopes, the heavier isotope may accumulate 
at the hot region or the cold region or not at all, depending on 
the nature of the intermolecular forces. In fact, the direction of 
separation may reverse as the temperature or relative concen- 
tration is changed. 

1 62 The Separation of Isotopes 

9.22. Such thermal diffusion effects were first used to separate 
isotopes by H. Clusius and G. Dickel in Germany in 1938. They 
built a vertical tube containing a heated wire stretched along 
the axis of the tube and producing a temperature difference of 
about 600 C. between the axis and the periphery. The effect was 
twofold. In the first place, the heavy isotopes (in the substances 
they studied) became concentrated near the cool outer wall, and 
in the second place, the cool gas on the outside tended to sink 
while the hot gas at the axis tended to rise. Thus thermal con- 
vection set up a countercurrent flow, and thermal diffusion 
caused the preferential flow of the heavy molecules outward 
across the interface between the two currents. 

9.23. The theory of thermal diffusion in gases is intricate 
enough; that of thermal diffusion in liquids is practically impos- 
sible. A separation effect does exist, however, and has been used 
successfully to separate the light and heavy uranium hexafluorides. 


9.24. In the introduction to this chapter we pointed out that 
there was some reason to hope that isotope separation might be 
accomplished by ordinary chemical reactions. It has in fact been 
found that in simple exchange reactions between compounds of 
two different isotopes the so-called equilibrium constant is not 
exactly one, and thus that in reactions of this type separation 
can occur. For example, in the catalytic exchange of hydrogen 
atoms between hydrogen gas and water, the water contains 
between three and four times as great a concentration of deu- 
terium as the hydrogen gas in equilibrium with it. With hydrogen 
and water vapor the effect is of the same general type but equilib- 
rium is more rapidly established. It is possible to adapt this 
method to a continuous countercurrent flow arrangement like 
that used in distillation, and such arrangements are actually in 
use for production of heavy water. The general method is well 
understood, and the separation effects are known to decrease in 
general with increasing molecular weight, so that there is but a 

The Separation of Isotopes 163 

small chance of applying this method successfully to heavy iso- 
topes like uranium. 


9.25. The electrolysis method of separating isotopes resulted 
from the discovery that the water contained in electrolytic cells 
used in the regular commercial production of hydrogen and 
oxygen has an increased concentration of heavy water molecules. 
A full explanation of the effect has not yet been worked out. 
Before the war practically the entire production of heavy hydro- 
gen was by the electrolysis method. By far the greatest production 
was in Norway, but enough for many experimental purposes had 
been made in the United States. 


9.26. The six methods of isotope separation we have described 
so far (diffusion, distillation, centrifugation, thermal diffusion, 
exchange reactions, and electrolysis) have all been tried with some 
degree of success on either uranium or hydrogen or both. Each 
of these methods depends on small differences in the average 
behavior of the molecules of different isotopes. Because an average 
is by definition a statistical matter, all such methods depending 
basically on average behavior are called statistical methods. 

9.27. With respect to the criteria set up for judging separation 
processes the six statistical methods are rather similar. In every 
case the separation factor is small so that many successive stages 
of separation are required. In most cases relatively large quantities 
of material can be handled in plants of moderate size. The hold- 
up and starting-time values vary considerably but are usually high. 
The similarity of the six methods renders it inadvisable to make 
final choice of method without first studying in detail the particu- 
lar isotope, production rate, etc., wanted. Exchange reaction 
and electrolysis methods are probably unsuitable in the case of 
uranium, and no distillation scheme for uranium has survived. 
All of the other three methods have been developed with varying 
degrees of success for uranium, but are not used for hydrogen. 


The Separation of Isotopes 


9.28. The existence of non-radioactive isotopes was first 
demonstrated during the study of the behavior of ionized gas 
molecules moving through electric and magnetic fields. It is just 







such fields that form the basis of the so-called mass spectrographic 
or electromagnetic method of separating isotopes. This method 
is the best available for determining the relative abundance of 
many types of isotope. The method is used constantly in checking 
the results of the uranium isotope separation methods we have 
already described. The reason the method is so valuable is that 
it can readily effect almost complete separation of the isotopes 
very rapidly and with small hold-up and short start-up time. If 

The Separation of Isotopes 165 

this is so, it may well be asked why any other method of separa- 
tion is considered. The answer is that an ordinary mass spectro- 
graph can handle only very minute quantities of material, usually 
of the order of fractions of a microgram per hour. 

9.29. To understand the reasons for this limitation in the yield, 
we shall outline the principle of operation of a simple type of 
mass spectrograph first used by A. J. Dempster in 1918. Such an 
instrument is illustrated schematically in the drawing on p. 164. 
The gaseous compound to be separated is introduced in the 
ion source, where some of its molecules are ionized in an electric 
discharge. Some of these ions go through the slit SL Between 
Si and 82 they are accelerated by an electric field which gives 
them all practically the same kinetic energy, thousands of times 
greater than their average thermal energy. Since they now all 
have practically the same kinetic energy, the lighter ions must 
have less momenta than the heavy ones. Entering the magnetic 
field at the slit 82 all the ions will move perpendicular to the 
magnetic field in semi-circular paths of radii proportional to 
their momenta. Therefore the light ions will move in smaller 
semicircles than the heavy, and with proper positioning of the 
collector, only the light ions will be collected. 

9.30. Postponing detailed discussion of such a separation 
device, we may point out the principal considerations that limit 
the amount of material that passes through it. They are three- 
fold: First, it is difficult to produce large quantities of gaseous 
ions. Second, a sharply limited ion beam is usually employed (as 
in the case shown) so that only a fraction of the ions produced 
are used. Third, too great densities of ions in a beam can cause 
space-charge effects which interfere with the separating action. 
Electromagnetic methods developed before 1941 had very high 
separation factors but very low yields and efficiencies. These were 
the reasons which before the summer of 1941 led the Uranium 
Committee to exclude such methods for large-scale separation 
of U-235. (See Paragraph 4:31.) Since that time it has been shown 
that the limitations are not insuperable. In fact, the first appreci- 
able-size samples of pure U-235 were produced by an electro- 
magnetic separator, as will be described in a later chapter. 

1 66 The Separation of Isotopes 


9.31. In addition to the isotope-separation methods described 
above, several other methods have been tried. These include the 
ionic mobility method, which, as the name implies, depends on 
the following fact: In an electrolytic solution two ions which are 
chemically identical but of different mass progress through the 
solution at different rates under the action of an electric field. 
However, the difference of mobility will be small and easily 
obscured by disturbing effects. A. K. Brewer of the Bureau of 
Standards reported that he was able to separate the isotopes of 
potassium by this method. Brewer also obtained some interesting 
results with an evaporation method. Two novel electromagnetic 
methods, the isotron and the ionic centrifuge, are described in 
Chapter XI. The isotron produced a number of fair-size samples 
of partly separated uranium. The ionic centrifuge also produced 
some uranium samples showing separation, but its action was 


9.32. In all the statistical methods of separating isotopes many 
successive stages of separation are necessary to get material that 
is 90 per cent or more U-235 or deuterium. Such a series of 
successive separating stages is called a cascade if the flow is 
continuous from one stage to the next. (A fractionating tower of 
separate plates such as has been described is an example of a 
simple cascade of separating units.) A complete analysis of the 
problems of a cascade might be presented in general terms. 
Actually it has been worked out by R. P. Feynman of Princeton 
and others for a certain type of electromagnetic separator and by 
K. Cohen and I. Kaplan of Columbia, by M. Benedict and A. M. 
Squires of the Kellex Corporation and others for diffusion proc- 
esses. At present we shall make only two points about multiple- 
stage or "cascade" plants. 

9.33. The first point is that there must be recycling. Con- 
sidering a U-235 separation plant, the material fed into any stage 

The Separation of Isotopes 167 

above the first has already been enriched in U-235. Part of this 
feed material may be further enriched in passing through the 
stage under consideration. The remainder will typically become 
impoverished but not so much impoverished as to be valueless. 
It must be returned to an earlier stage and recycled. Even the 
impoverished material from the first (least enriched) stage may be 
worth recycling; some of the U-235 it still contains may be re- 
covered (stripped). 

9.34. The second point is that the recycling problem changes 
greatly at the higher (more enriched) stages. Assuming steady 
stage operation, we see that the net flow of uranium through the 
first stage must be at least 140 times as great as through the last 
stage. The net flow in any given stage is proportional to the rela- 
tive concentration of U-238 and thus decreases with the number of 
stages passed. Since any given sample of material is recycled many 
times, the amount of material processed in any stage is far greater 
than the net flow through that stage but is proportional to it. 

9.35. We mention these points to illustrate a phase of the 
separation problem that is not always obvious, namely, that the 
separation process which is best for an early stage of separation 
is not necessarily best for a later stage. Factors such as those we 
have mentioned differ not only from stage to stage but from 
process to process. For example, recycling is far simpler in a 
diffusion plant than in an electromagnetic plant. A plant com- 
bining two or more processes may well be the best to accomplish 
the overall separation required. In the lower (larger) stages the 
size of the equipment and the power required for it may deter- 
mine the choice of process. In the higher (smaller) stages these 
factors are outweighed by convenience of operation and hold-up 
time, which may point to a different process. 


9.36. The next two chapters are devoted to descriptions of the 
three methods used for large-scale separation of the uranium 
isotopes. These are the only isotope-separation plants that have 

168 The Separation of Isotopes 

turned out to be of major importance to the project up to the 
present time. At an earlier stage it seemed likely that the centri- 
fuge might be the best method for separating the uranium isotopes 
and that heavy water would be needed as a moderator. We shall 
describe briefly the centrifuge pilot plant and the heavy water 
production plants. 


9.37. Two methods were used for the concentration of deu- 
terium. These were the fractional distillation of water and the 
hydrogen-water exchange reaction method. 

9.38. The first of these follows well established fractional 
distillation methods except that very extensive distillation is 
required because of the slight difference in boiling point of light 
and heavy water. Also, because of this same small difference, the 
amount of steam required is very large. The method is very 
expensive because of these factors, but plants could be con- 
structed with a minimum of development work. Plants were 
started by du Pont in January 1943, and were put into operation 
about January 1 944. 

9.39. The second method for the preparation of heavy water 
depends upon the catalytic exchange of deuterium between 
hydrogen gas and water. When such an exchange is established 
by catalysts, the concentration of the deuterium in the water is 
greater than that in the gas by a factor of about three as we have 
already seen. 

9.40. In this process water is fed into a tower and flows counter- 
currently to hydrogen and steam in an intricate manner. At the 
bottom of the tower the water is converted to hydrogen gas and 
oxygen gas in electrolytic cells and the hydrogen is fed back to 
the bottom of the tower mixed with steam. This steam and 
hydrogen mixture passes through beds of catalyst and bubbles 
through the downflowing water. Essentially, part of the deu- 
terium originally in the hydrogen concentrates in the steam and 
then is transferred to the downflowing water. The actual plant 
consists of a cascade of towers with the largest towers at the feed 

The Separation of Isotopes 169 

end and the smallest towers at the production end. Such a cascade 
follows the same general principle as those discussed above in 
connection with separation problems in general. This process 
required the securing of very active catalysts for the exchange 
reactions. The most effective catalyst of this type was discovered 
by H. S. Taylor at Princeton University, while a second, less 
active catalyst was discovered by A. von Grosse. In the develop- 
ment of these catalysts R. H. Crist of Columbia University made 
the necessary determinations of physical constants and H. R. 
Arnold of du Pont did the development work on one of the 

9.41. This process was economical in operation. The plant was 
placed at the works of the Consolidated Mining & Smelting Co., 
at Trail, British Columbia, Canada, because of the necessity of 
using electrolytic hydrogen. The construction of the plant was 
under the direction of E. V. Murphree and F. T. Barr of the 
Standard Oil Development Co. 


9.42. For a long time in the early days of the project the gase- 
ous diffusion method and the centrifuge method were considered 
the two separation methods most likely to succeed with uranium. 
Both were going to be difficult to realize on a large scale. After 
the reorganization in December 1941 research and development 
on the centrifuge method continued at the University of Virginia 
and at the Standard Oil Development Company's laboratory at 
Bayway. To make large centrifuges capable of running at very 
high speeds was a major task undertaken by the Westinghouse 
Electric and Manufacturing Company of East Pittsburgh. 

9.43. Because of the magnitude of the engineering problems 
involved, no large-scale production plant was ever authorized 
but a pilot plant was authorized and constructed at Bayway. 
It was operated successfully and gave approximately the degree 
of separation predicted by theory. This plant was later shut down 
and work on the centrifuge method was discontinued. For this 

170 The Separation of Isotopes 

reason no further discussion of the centrifuge method is given in 
this report. 


9.44. The most important methods of isotope separation that 
have been described were known in principle and had been 
reduced to practice before the separation of uranium isotopes 
became of paramount importance. They had not been applied 
to uranium except for the separation of a few micrograms, and 
they had not been applied to any substance on a scale comparable 
to that now required. But the fundamental questions were of 
costs, efficiency, and time, not of principle; in other words, the 
problem was fundamentally technical, not scientific. The plu- 
tonium production problem did not reach a similar stage until 
after the first self-sustaining chain-reacting pile had operated and 
the first microgram amounts of plutonium had been separated. 
Even after this stage many of the experiments done on the 
plutonium project were of vital interest for the military use either 
of U-235 or plutonium and for the future development of nuclear 
power. As a consequence, the plutonium project has continued 
to have a more general interest than the isotope separation proj- 
ects. Many special problems arose in the separation projects 
which were extremely interesting and required a high order of 
scientific ability for their solution but which must still be kept 
secret. It is for such reasons that the present non-technical report 
has given first emphasis to the plutonium project and will give 
less space to the separation projects. This is not to say that the 
separation problem was any easier to solve or that its solution 
was any less important. 


9.45. Except in electromagnetic separators, isotope separation 
depends on small differences in the average behavior of molecules. 
Such effects are used in six "statistical" separation methods: 
(1) gaseous diffusion, (2) distillation, (3) centrifugation, (4) 

The Separation of Isotopes 171 

thermal diffusion, (5) exchange reactions, (6) electrolysis. 
Probably only (1), (3), and (4) are suitable for uranium; (2), 
(5), and (6) are preferred for the separation of deuterium from 
hydrogen. In all these "statistical" methods the separation factor 
is small so that many stages are required, but in the case of each 
method large amounts of material may be handled. All these 
methods had been tried with some success before 1940; however, 
none had been used on a large scale and none had been used for 
uranium. The scale of production by electromagnetic methods 
was even smaller but the separation factor was larger. There 
were apparent limitations of scale for the electromagnetic 
method. There were presumed to be advantages in combining 
two or more methods because of the differences in performance 
at different stages of separation. The problem of developing any 
or all of these separation methods was not a scientific one of 
principle but a technical one of scale and cost. These develop- 
ments can therefore be reported more briefly than those of the 
plutonium project although they are no less important. A pilot 
plant was built using centrifuges and operated successfully. No 
large-scale plant was built. Plants were built for the production 
of heavy water by two different methods. 





10.1. It was in February 1940 that small amounts of concen- 
trated fractions of the three uranium isotopes of masses 234, 235, 
and 238 were obtained by A. O. Nier using his mass spectrometer 
and were turned over to E. T. Booth, A. von Grosse, and J. R. 
Dunning for investigation with the Columbia University cyclo- 
tron. These men soon demonstrated that U-235 was the isotope 
susceptible to fission by thermal neutrons. It was natural, there- 
fore, that this group, under the leadership of Dunning, became 
more interested than ever in the large-scale separation of the 
uranium isotopes. 

10.2. The diffusion method was apparently first seriously 
reviewed by Dunning in a memorandum to G. B. Pegram, which 
was sent to L. J. Briggs in the fall of 1940. This memorandum 
summarized preliminary investigations that had been carried 
on by E. T. Booth, A. von Grosse and J. R. Dunning. Work was 
accelerated in 1941 with financial help provided by a contract 
that H. C. Urey had recived from the Navy for the study of 
isotope separation principally by the centrifuge method. During 
this period F. G. Slack of Vanderbilt University and W. F. 
Libby of the University of California joined the group. An OSRD 
contract (OEMsr-106) calling specifically for diffusion studies 
went into effect on July 1, 1941, and ran for a year. The work con- 
tinued on an expanding scale under a series of OSRD and Army 
contracts through the spring of 1945. Up until May 1943 Dun- 
ning was in immediate charge of this work; Urey was in charge 
of statistical methods in general. From that time until February 
1945 Urey was in direct charge of the Columbia part of the 


Diffusion Separation 173 

diffusion work, with Dunning continuing as director of one of 
the principal divisions. On March 1, 1945, the laboratory was 
taken over from Columbia by Carbide and Carbon Chemicals 
Corporation. Early in 1 942, at the suggestion of E. V. Murphree, 
the M. W. Kellogg Company was brought in to develop plans 
for large-scale production of diffusion-plant equipment and 
eventually to build a full-scale plant. To carry out this under- 
taking, a new subsidiary company was formed called the Kellex 
Corporation. In January 1943, Carbide and Carbon Chemicals 
Corporation was given the responsibility for operating the plant. 

10.3. As stated in Chapter IV, by the end of 1941 the possi- 
bility of separating the uranium hexafluorides had been demon- 
strated in principle by means of a single-stage diffusion unit 
employing a porous barrier (for example, a barrier made by 
etching a thin sheet of silver-zinc alloy with hydrochloric acid) . 
A considerable amount of work on barriers and pumps had also 
been done but no answer entirely satisfactory for large-scale 
operation had been found. Also, K. Cohen had begun a series 
of theoretical studies, to which reference has already been made, 
as to what might be the best way to use the diffusion process, 
i.e., as to how many stages would be required, what aggregate 
area of barrier would be needed, what volume of gas would have 
to be circulated, etc. Theoretical studies and process develop- 
ment by M. Benedict added much to knowledge in this field and 
served as the basis of design of the large plant. 

10.4. Reports received from the British, and the visit by the 
British group in the winter of 1941-1942, clarified a number of 
points. At that time the British were planning a diffusion sepa- 
ration plant themselves so that the discussions with F. Simon, R. 
Peierls, and others were particularly valuable. 



10.5. As was explained in the last chapter, the rate of diffusion 
of a gas through an ideal porous barrier is inversely proportional 

174 Diffusion Separation 

to the square root of its molecular weight. Thus if a gas consisting 
of two isotopes starts to diffuse through a barrier into an evacu- 
ated vessels, the lighter isotope (of molecular weight MI) diffuses 
more rapidly than the heavier (of molecular weight M2). The 
result, for a short period of time, at least, is that the relative con- 
centration of the lighter isotope is greater on the far side of the 
barrier than on the near side. But if the process is allowed to 
continue indefinitely, equilibrium will become established and 
the concentrations will become identical on both sides of the 
barrier. Even if the diffusate gas (the gas which has passed through 
the barrier) is drawn away by a pump, the relative amount of the 
heavy isotope passing through the barrier will increase since 
the light isotope on the near side of the barrier has been depleted 
by the earlier part of the diffusion. 

10.6. For a single diffusion operation, the increase in the rela- 
tive concentration of the light isotope in the diffused gas com- 
pared to the feed gas can be expressed in terms of the separation 
factor r or the enrichment factor, r 1, both defined in para- 
graph 9.8 of the last chapter. A rather simple equation can be 
derived which gives r 1 in terms of the molecular weights and 
the fraction of the original gas which has diffused. If this fraction 
is very small, the equation reduces to r = a, the "ideal separation 
factor" of paragraph 9.14. If the fraction diffused is appreciable, 
the equation shows the expected diminution in separation. For 
example, if half the gas diffuses, r 1 = .69 (a 1), or for 
uranium hexafluoride r = 1 .003 compared to the value of 1 .0043 
when a very small fraction of the original gas has diffused. 


10.7. To separate the uranium isotopes, many successive 
diffusion stages (i.e., a cascade) must be used since a 1.0043 
for U 235 F 6 and U 238 F 6 , a possible gas for uranium separation. 
Studies by Cohen and others have shown that the best flow 
arrangement for the successive stages is that in which half the 
gas pumped into each stage diffuses through the barrier, the 
other (impoverished) half being returned to the feed of the next 

Diffusion Separation 175 

lower stage. For such an arrangement, as we have seen, the ideal 
separating effect between the feed and output of a single stage 
is 0.69(a 1). This is often called e, the "overall enrichment 
per stage." For the uranium hexafluorides, = 0.003, in theory; 
but it is somewhat less in practice as a result of "back diffusion," 
of imperfect mixing on the high pressure side, and of imperfec- 
tions in the barrier. The first experimental separation of the 
uranium hexafluorides (by E. T. Booth, H. C. Paxton, and C. B. 
Slade) gave results corresponding to = 0.0014. If one desires 
to produce 99 per cent pure U 235 F 6 , and if one uses a cascade 
in which each stage has a reasonable overall enrichment factor, 
then it turns out that roughly 4,000 stages are required. 


10.8. Of the gas that passes through the barrier of any given 
stage, only half passes through the barrier of the next higher 
stage, the other half being returned to an earlier stage. Thus 
most of the material that eventually emerges from the cascade 
has been recycled many times. Calculation shows that for an 
actual uranium-separation plant it may be necessary to force 
through the barriers of the first stage 100,000 times the volume 
of gas that comes out the top of the cascade (i.e., as desired 
product U 235 F 6 ). The corresponding figures for higher stages 
fall rapidly because of reduction in amount of unwanted material 
(U 238 F 6 ) that is carried along. 



10.9. By the time of the general reorganization of the atomic- 
bomb project in December 1 941 , the theory of isotope separation 
by gaseous diffusion was well understood. Consequently it was 
possible to define the technical problems that would be en- 
countered in building a large-scale separation plant. The decisions 
as to scale and location of such plant were not made until the 
\vinter of 1942-1943, that is, about the same time as the corre- 

176 Diffusion Separation 

spending decisions were being made for the plutonium produc- 
tion plants. 


10.10. The general objective of the large-scale gaseous diffusion 
plant was the production each day of a specified number of 
grams of uranium containing of the order of ten times as much 
U-235 as is present in the same quantity of natural uranium. 
However, it was apparent that the plant would be rather flexible 
in operation, and that considerable variations might be made in 
the degree of enrichment and yield of the final product. 


10.11. Uranium hexafluoride has been mentioned as a gas 
that might be suitable for use in the plant as "process gas"; not 
the least of its advantages is that fluorine has only one isotope so 
that the UFe molecules of any given uranium isotope all have the 
same mass. This gas is highly reactive and is actually a solid at 
room temperature and atmospheric pressure. Therefore the study 
of other gaseous compounds of uranium was urgently undertaken. 
As insurance against failure in this search for alternative gases, 
it was necessary to continue work on uranium hexafluoride, as 
in devising methods for producing and circulating the gas. 


10.12. The number of stages required in the main cascade of 
the plant depended only on the degree of enrichment desired 
and the value of overall enrichment per stage attainable with 
actual barriers. Estimates were made which called for several 
thousand stages. There was also to be a "stripping" cascade of 
several hundred stages, the exact number depending on how 
much unseparated U-235 could economically be allowed to go 
to waste. 


10.13. We have seen that the total value of gas that must 
diffuse through the barriers is very large compared to the volume 

Diffusion Separation 177 

of the final product. The rate at which the gas diffuses through 
unit area of barrier depends on the pressure difference on the 
two sides of the barrier and on the porosity of the barrier. Even 
assuming full atmospheric pressure on one side and zero pressure 
on the other side, and using an optimistic figure for the porosity, 
calculations showed that many acres of barrier would be needed 
in the large-scale plant. 


10.14. At atmospheric pressure the mean free path of a mole- 
cule is of the order of a ten-thousandth of a millimeter or one 
tenth of a micron. To insure true "diffusive" flow of the gas, the 
diameter of the myriad holes in the barrier must be less than one 
tenth the mean free path. Therefore the barrier material must 
have almost no holes which are appreciably larger than 0.01 
micron (4 X 10~ 7 inch), but must have billions of holes of this 
size or smaller. These holes must not enlarge or plug up as the 
result of direct corrosion or dust coming from corrosion elsewhere 
in the system. The barrier must be able to withstand a pressure 
"head" of one atmosphere. It must be amenable to manufacture 
in large quantities and with uniform quality. By January 1 942, a 
number of different barriers had been made on a small scale and 
tested for separation factor and porosity. Some were thought to 
be very promising, but none had been adequately tested for actual 
large-scale production and plant use. 


10.15. In any given stage approximately half of the material 
entering the stage passes through the barrier and on to the next 
higher stage, while the other half passes back to the next lower 
stage. The diffused half is at low pressure and must be pumped to 
high pressure before feeding into the next stage. Even the undif- 
fused portion emerges at somewhat lower pressure than it entered 
and cannot be fed back to the lower stage without pumping. 
Thus the total quantity of gas per stage (comprising twice the 

178 Diffusion Separation 

amount which flows through the barrier) has to be circulated by 
means of pumps. 

10.16. Since the flow of gas through a stage varies greatly with 
the position of the stage in the cascade, the pumps also vary 
greatly in size or number from stage to stage. The type and 
capacity of the pump required for a given stage depends not 
only on the weight of gas to be moved but on the pressure rise 
required. Calculations made at this time assumed a fore pressure 
of one atmosphere and a back pressure (i.e., on the low pressure 
side of the barrier) of one tenth of an atmosphere. It was esti- 
mated that thousands of pumps would be needed and that 
thousands of kilowatts would be required for their operation. 
Since an unavoidable concomitant of pumping gas is heating 
it, it was evident that a large cooling system would have to be 
provided. By early 1942, a good deal of preliminary work had 
been done on pumps. Centrifugal pumps looked attractive in 
spite of the problem of sealing their shafts, but further experi- 
mental work was planned on completely sealed pumps of various 


10.17. It was clear that the whole circulating system com- 
prising pumps, barriers, piping, and valves would have to be 
vacuum tight. If any lubricant or sealing medium is needed in 
the pumps, it should not react with the process gas. In fact none 
of the materials in the system should react with the process gas 
since such corrosion would lead not only to plugging of the 
barriers and various mechanical failures but also to absorption 
(i.e., virtual disappearance) of uranium which had already been 
partially enriched. 


10.18. In an ideal cascade, the pumping requirements change 
from stage to stage. In practice it is not economical to provide a 
different type of pump for every stage. It is necessary to deter- 
mine how great a departure from the ideal cascade (i.e., what 

Diffusion Separation 179 

minimum number of pump types) should be employed in the 
interest of economy of design, repair, etc. Similar compromises 
are used for other components of the cascade. 


10.19. When first started, the plant must be allowed to run 
undisturbed for some time, until enough separation has been 
effected so that each stage contains gas of appropriate enrich- 
ment. Only after such stabilization is attained is it desirable to 
draw off from the top stage any of the desired product. Both 
the amount of material involved (the hold-up) and the time 
required (the start-up time) are great enough to constitute major 
problems in their own right. 


10.20. It was apparent that there would be only three types of 
material loss in the plant contemplated, namely: loss by leakage, 
loss by corrosion (i.e., chemical combination and deposition), 
and loss in plant waste. It was expected that leakage could be 
kept very small and that after an initial period of operation 
loss from corrosion would be small. The percentage of material 
lost in plant waste would depend on the number of stripping 


10.21. Questions as to how the barrier material was to be used 
(whether in tubes or sheets, in large units or small units), how 
mixing was to be effected, and what controls and instruments 
would be required were still to be decided. There was little 
reason to expect them to be unanswerable, but there was no 
doubt that they would require both theoretical and experimental 


10.22. By 1942 the theory of isotope separation by gaseous 
diffusion had been well worked out, and it became clear that a 

180 Diffusion Separation 

very large plant would be required. The major equipment items 
in this plant were diffusion barriers and pumps. Neither the 
barriers nor the pumps which were available at that time had 
been proved generally adequate. Therefore the further develop- 
ment of pumps and barriers was especially urgent. There were 
also other technical problems to be solved, these involving cor- 
rosion, vacuum seals, and instrumentation. 


10.23. As we mentioned at the beginning of this chapter, the 
diffusion work was initiated by J. R. Dunning. The work was 
carried on under OSRD auspices at Columbia University until 
May 1, 1943, when it was taken over by the Manhattan District. 
In the summer of 1943 the difficulties encountered in solving 
certain phases of the project led to a considerable expansion, 
particularly of the chemical group. H. C. Urey, then director 
of the work, appointed H. S. Taylor of Princeton associate 
director and added E. Mack, Jr. of Ohio State, G. M. Murphy 
of Yale, and P. H. Emmett of Johns Hopkins to the senior staff. 
Most of the work was moved out of the Columbia laboratories 
to a large building situated near by. The chemists at Princeton 
who had been engaged in heavy water studies were assigned some 
of the barrier research problems. Early in 1944, L. M. Currie 
of the National Carbon Company became another associate 
director to help Urey in his liaison and administrative work. 

10.24. As has been mentioned, the M. W. Kellogg Company 
was chosen early in 1942 to plan the large scale plant. For this 
purpose Kellogg created a special subsidiary called The Kellex 
Corporation, with P. C. Leith as executive in charge and tech- 
nical head and, responsible to him, A. L. Baker as Project Man- 
ager, and J. H. Arnold as Director of Research and Development. 
The new subsidiary carried on research and development in its 
Jersey City laboratories and in the laboratory building referred 
to in the paragraph above; developed the process and engineering 
designs; and procured materials for the large-scale plant and 
supervised its construction. The plant was constructed by the 

Diffusion Separation 181 

J. A. Jones Construction Company, Incorporated, of Charlotte, 
North Carolina. 

10.24-a. The Kellex Corporation, unlike conventional indus- 
trial firms, was a cooperative of scientists, engineers and adminis- 
trators recruited from essentially all branches of industry and 
gathered for the express purpose of carrying forward this one job. 
Service was on a voluntary basis, individuals prominent in indus- 
try freely relinquishing their normal duties and responsibilities 
to devote full time to Kellex activities. As their respective tasks 
are being completed these men are returning to their former 
positions in industry. 

10.25. In January 1943, Carbide and Carbon Chemicals 
Corporation were chosen to be the operators of the completed 
plant. Their engineers soon began to play a large role not only 
in the planning and construction but also in the research work. 

PRODUCTION, 1942 TO 1945 


10.26. Even before 1942, barriers had been developed that 
were thought to be satisfactory. However, the barriers first 
developed by E. T. Booth, H. C. Paxton, and C. B. Slade were 
never used on a large scale because of low mechanical strength 
and poor corrosion resistance. In 1942, under the general super- 
vision of Booth and F. G. Slack and with the cooperation of 
various scientists including F. C. Nix of the Bell Telephone 
Laboratories, barriers of a different type were produced. At 
one time, a barrier developed by E. O. Norris and E. Adler 
was thought sufficiently satisfactory to be specified for plant use. 
Other barriers were developed by combining the ideas of 
several men at the Columbia laboratories (by now christened the 
SAM Laboratories), Kellex, Bell Telephone Laboratories, Bake- 
lite Corporation, Houdaille-Hershey Corporation, and others. 
The first specimens of the type of barrier selected for general use 
in the plant were prepared by C. A. Johnson of Kellex, and the 

182 Diffusion Separation 

barrier was perfected under the general supervision of H. S. 
Taylor. One modification of this barrier developed by the SAM 
Laboratories represented a marked improvement in quality and 
is being used in a large number of stages of the plant. By 1945 the 
problem was no longer one of barely meeting minimum specifica- 
tions, but of making improvements resulting in greater rate of 
output or greater economy of operation. 

10.27. Altogether the history of barrier development reminds 
the writer of the history of the "canning" problem of the plu- 
tonium project. In each case the methods were largely cut and 
try, and satisfactory or nearly satisfactory solutions were repeat- 
edly announced; but in each case a really satisfactory solution 
was not found until the last minute and then proved to be far 
better than had been hoped. 


10.28. The early work on pumps was largely under the super- 
vision of H. A. Boorse of Columbia University. When Kellex came 
into the picture in 1942, its engineers, notably G. W. Watts, 
J. S. Swearingen and O. C. Brewster, took leading positions in 
the development of pumps and seals. It must be remembered 
that these pumps are to be operated under reduced pressure, must 
not leak, must not corrode, and must have as small a volume as 
possible. Many different types of centrifugal blower pumps and 
reciprocating pumps were tried. In one of the pumps for the 
larger stages, the impeller is driven through a coupling contain- 
ing a very novel and ingenious type of seal. Another type of pump 
is completely enclosed, its centrifugal impeller and rotor being 
run from outside, by induction. 


10.29. As in the plutonium problem, so here also, there were 
many questions of corrosion, etc., to be investigated. New coolants 
and lubricants were developed by A. L. Henne and his associ- 
ates, by G. H. Cady, by W T . T. Miller and his co-workers, by 
E. T. McBee and his associates, and by scientists of various 

Diffusion Separation 183 

corporations including Hooker Electrochemical Co., the du Pont 
Co. and the Harshaw Chemical Co. The research and develop- 
ment and plant requirements for these materials and other special 
chemicals were coordinated by R. Rosen, first under OSRD and 
later for Kellex. Methods of pretreating surfaces against corrosion 
were worked out. Among the various instruments designed or 
adapted for project use, the mass spectrograph deserves special 
mention. The project was fortunate in having the assistance of 
A. O. Nier of the University of Minnesota and later of Kellex 
whose mass spectrograph methods of isotope analysis were suffi- 
ciently advanced to become of great value to the project, as in 
analyzing samples of enriched uranium. Mass spectrographs 
were also used in pretesting parts for vacuum leaks and for 
detecting impurities in the process gas in the plant. 


10.30. Strictly speaking, there was no pilot plant. That is to 
say, there was no small-scale separation system set up using the 
identical types of blowers, barriers, barrier mountings, cooling, 
etc., that were put into the main plant. Such a system could 
not be set up because the various elements of the plant were not 
all available prior to the construction of the plant itself. To pro- 
ceed with the construction of the full-scale plant under these 
circumstances required foresight and boldness. 

10.31. There was, however, a whole series of so-called pilot 
plants which served to test various components or groups of 
components of the final plant. Pilot plant No. 1 was a 12-stage 
plant using a type of barrier rather like that used in the large- 
scale plant, but the barrier material was not fabricated in the 
form specified for the plant and the pumps used were sylphon- 
sealed reciprocating pumps, not centrifugal pumps. Work on this 
plant in 1943 tested not only the barriers and general system of 
separation but gave information about control valves, pressure 
gauges, piping, etc. Pilot plant No. 2, a larger edition of No. 1 
but with only six stages, was used in late 1943 and early 1944, 
particularly as a testing unit for instruments. Pilot plant No. 3a, 

184 Diffusion Separation 

using centrifugal blowers and dummy diffusers, was also intended 
chiefly for testing instruments. Pilot plant No. 3b was a real 
pilot plant for one particular section of the large-scale plant. Pilot 
plants using full-scale equipment at the plant site demonstrated 
the vacuum tightness, corrosion resistance and general operability 
of the equipment. 


10.32. In December 1942, the Kellogg Company was author- 
ized to proceed with preliminary plant design and in January 
1943 the construction of a plant was authorized. 


10.33. As stated in an earlier chapter, a site in the Tennessee 
Valley had originally been chosen for all the Manhattan District 
plants, but the plutonium plant was actually constructed else- 
where. There remained the plutonium pilot plant already 
described, the gaseous diffusion plant, the electromagnetic 
separation plant (see Chapter XI), and later the thermal dif- 
fusion plant which were all built in the Tennessee Valley at the 
Clinton site, known officially as the Clinton Engineer Works. 

10.34. This site was examined by Colonel Marshall, Colonel 
Nichols, and representatives of Stone and Webster Engineering 
Corporation in July 1942, and its acquisition was recommended. 
This recommendation was endorsed by the OSRD S-l Executive 
Committee at a meeting in July 1942. Final approval was given 
by Major General L. R. Groves after personal inspection of the 
70-square-mile site. In September 1942, the first steps were taken 
to acquire the tract, which is on the Clinch River about thirty 
miles from Knoxville, Tennessee, and eventually considerably 
exceeded 70 square miles. The plutonium pilot plant is located 
in one valley, the electromagnetic separation plant in an adjoin- 
ing one, and the diffusion separation plant in a third. 

10.35. Although the plant and site development at Hanford 
is very impressive, it is all under one company dealing with but 
one general operation so that it is in some respects less interesting 

Diffusion Separation 185 

than Clinton, which has a great multiplicity of activity. To 
describe the Clinton site, with its great array of new plants, its 
new residential districts, new theatres, new school system, seas 
of mud, clouds of dust, and general turmoil is outside the scope 
of this report. 


10.36. Construction of the steam power plant for the diffusion 
plant began on June 1, 1943. It is one of the largest such power 
plants ever built. Construction of other major buildings and 
plants started between August 29, 1943 and September 10, 1943. 


10.37. Unlike Hanford, the diffusion plant consists of so many 
more or less independent units that it was put into operation 
section by section, as permitted by progress in constructing and 
testing. Thus there was no dramatic start-up date nor any unto- 
ward incident to mark it. The plant was in successful operation 
before the summer of 1945. 

10.38. For the men working on gaseous diffusion it was a long 
pull from 1940 to 1945, not lightened by such exciting half-way 
marks as the first chain-reacting pile at Chicago. Perhaps more 
than any other group in the project, those who have worked on 
gaseous diffusion deserve credit for courage and persistence as 
well as scientific and technical ability. For security reasons, we 
have not been able to tell how they solved their problems even 
in many cases found several solutions, as insurance against 
failure in the plant. It has been a notable achievement. In these 
five years there have been periods of discouragement and pessi- 
mism. They are largely forgotten now that the plant is not only 
operating but operating consistently, reliably, and with a per- 
formance better than had been anticipated. 


10.39. Work at Columbia University on the separation of 
isotopes by gaseous diffusion began in 1940, and by the end of 

186 Diffusion Separation 

1942 the problems of large-scale separation of uranium by this 
method had been well defined. Since the amount of separation 
that could be effected by a single stage was very small, several 
thousand successive stages were required. It was found that 
the best method of connecting the many stages required extensive 
recycling so that thousands of times as much material would pass 
through the barriers of the lower stages as would ultimately 
appear as product from the highest stage. 

10.40. The principal problems were the development of satis- 
factory barriers and pumps. Acres of barrier and thousands of 
pumps were required. The obvious process gas was uranium hexa- 
fluoride for which the production and handling difficulties were 
so great that a search for an alternative was undertaken. Since 
much of the separation was to be carried out at low pressure, 
problems of vacuum technique arose, and on a previously 
unheard-of scale. Many problems of instrumentation and control 
were solved; extensive use was made of various forms of mass 

10.41. The research was carried out principally at Columbia 
under Dunning and Urey. In 1942, the M. W. Kellogg Company 
was chosen to develop the process and equipment and to design 
the plant and set up the Kellex Corporation for the purpose. The 
plant was built by the J. A. Jones Construction Company. The 
Carbide and Carbon Chemicals Corporation was selected as 
operating company. 

10.42. A very satisfactory barrier was developed although the 
final choice of barrier type was not made until the construction 
of the plant was well under way at Clinton Engineer Works in 
Tennessee. Two types of centrifugal blower were developed to 
the point where they could take care of the pumping require- 
ments. The plant was put into successful operation before the 
summer of 1945. 





11.1. In Chapter IV we said that the possibility of large-scale 
separation of the uranium isotopes by electromagnetic means was 
suggested in the fall of 1941 by E. O. Lawrence of the University 
of California and H. D. Smyth of Princeton University. In Chap- 
ter IX we described the principles of one method of electromag- 
netic separation and listed the three limitations of that method: 
difficulty of producing ions, limited fraction of ions actually used, 
and space charge effects. 

11.2. By the end of December 1941, when the reorganization 
of the whole uranium project was effected, Lawrence had already 
obtained some samples of separated isotopes of uranium and in 
the reorganization he was officially placed in charge of the prepa- 
ration of further samples and the making of various associated 
physical measurements. However, just as the Metallurgical 
Laboratory very soon shifted its objective from the physics of 
the chain reaction to the large-scale production of plutonium, 
the objective of Lawrence's division immediately shifted to the 
effecting of large-scale separation of uranium isotopes by electro- 
magnetic methods. This change was prompted by the success 
of the initial experiments at California and by the development 
at California and at Princeton of ideas on other possible methods. 
Of the many electromagnetic schemes suggested, three soon were 
recognized as being the most promising: the "calutron" mass 
separator, the magnetron-type separator later developed into 
the "ionic centrifuge," and the "isotron" method of ''bunching" 
a beam of ions. The first two of these approaches were followed 
at California and the third at Princeton. After the first few 


188 Electromagnetic Separation 

months, by far the greatest effort was put on the calutron, but 
some work on the ionic centrifuge was continued at California 
during the summer of 1942 and was further continued by J. 
Slepian at the Westinghouse laboratories in Pittsburgh on a 
small scale through the winter of 1944-1945. Work on the isotron 
was continued at Princeton until February 1943, when most of 
the group was transferred to other work. Most of this chapter 
will be devoted to the calutron since that is the method that has 
resulted in large-scale production of U-235. A brief description 
will also be given of the thermal diffusion plant built to provide 
enriched feed material for the electromagnetic plant. 

11.3. Security requirements make it impossible here as for 
other parts of the project to present many of the most interesting 
technical details. The importance of the development is con- 
siderably greater than is indicated by the amount of space which 
is given it here. 



11.4. A. O. Nier's mass spectrograph was set up primarily to 
measure relative abundances of isotopes, not to separate large 
samples. Using vapor from uranium bromide Nier had prepared 
several small samples of separated isotopes of uranium, but his 
rate of production was very low indeed, since his ion current 
amounted to less than one micro-ampere. (A mass spectrograph 
in which one micro-ampere of normal uranium ions passes 
through the separating fields to the collectors will collect about 
one microgram of U-235 per 16-hour day.) The great need of 
samples of enriched U-235 for nuclear study was recognized 
early by Lawrence, who decided to see what could be done with 
the help of the 37-inch (cyclotron) magnet at Berkeley. The 
initial stages of this work were assisted by a grant from the Re- 
search Corporation of New York, which was later repaid. Be- 
ginning January 1, 1942, the entire support came from the OSRD 
through the S-l Committee. Later, as in other parts of the 

Electromagnetic Separation 189 

uranium project, the contracts were taken over by the Manhattan 

11.5. At Berkeley, after some weeks of planning, the 37-inch 
cyclotron was dismantled on November 24, 1941, and its magnet 
was used to produce the magnetic field required in what came 
to be called a "calutron" (a name representing a contraction of 
"California University cyclotron"). An ion source consisting of 
an electron beam traversing the vapor of a uranium salt was set 
up corresponding to the ion source shown in the drawing in 
Chap. IX, p. 164. Ions were then accelerated to the slit 82 through 
which they passed into the separating region where the magnetic 
field bent their paths into semicircles terminating at the collector 
slit. By December 1, 1941, molecular ion beams from the residual 
gas were obtained, and shortly thereafter the beam consisting 
of singly charged uranium ions (U+) was brought up to an 
appreciable strength. It was found that a considerable proportion 
of the ions leaving the source were U+ ions. For the purpose of 
testing the collection of separated samples, a collector with two 
pockets was installed, the two pockets being separated by a 
distance appropriate to the mass numbers 235 and 238. Two 
small collection runs using U+ beams of low strength were made 
in December, but subsequent analyses of the samples showed 
only a small separation factor. By the middle of January 1942, 
a run had been made with a reasonable beam strength and an 
aggregate flow or through-put of appreciable amount which 
showed a much improved separation factor. By early February 
1 942, beams of much greater strength were obtained, and Law- 
rence reported that good separation factors were obtainable with 
such beams. By early March 1942, the ion current had been 
raised still further. These results tended to bear out Lawrence's 
hopes that space charge could be neutralized by ionization of the 
residual gas in the magnet chamber. 


11.6. By this time it was clear that the calutron was potentially 
able to effect much larger scale separations than had ever before 

190 Electromagnetic Separation 

been approached by an electromagnetic method. It was evi- 
dently desirable to explore the whole field of electromagnetic 
separation. With this end in view, Lawrence mobilized his group 
at the Radiation Laboratory of the University of California at 
Berkeley and began to call in others to help. Among those 
initially at Berkeley were D. Cooksey, P. C. Aebersold, W. M. 
Brobeck, F. A. Jenkins, K. R. MacKenzie, W. B. Reynolds, 
D. H. Sloan, F. Oppenheimer, J. G. Backus, B. Peters, A. C. 
Helmholz, T. Finkelstein, and W. E. Parkins, Jr. Lawrence 
called back some of his former students, including R. L. Thornton, 
J. R. Richardson, and others. Among those working at Berkeley 
for various periods were L. P. Smith from Cornell, E. U. Condon 
and J. Slepian from Westinghouse, and I. Langmuir and 
K. H. Kingdon from General Electric. During this early period 
J. R. Oppenheimer was still at Berkeley and contributed some 
important ideas. In the fall of 1 943 the group was further strength- 
ened by the arrival of a number of English physicists under the 
leadership of M. L. Oliphant of the University of Birmingham. 

11.7. Initially a large number of different methods were con- 
sidered and many exploratory experiments were performed. The 
main effort, however, soon became directed towards the develop- 
ment of the calutron, the objective being a high separation factor 
and a large current in the positive ion beam. 


11.8. Of the three apparent limitations listed in the first 
paragraph difficulty of producing ions, limited fraction of ions 
actually used, and space charge effects only the last had yielded 
to the preliminary attack. Apparently space charge in the neigh- 
borhood of the positive ion beam could be nullified to a very 
great extent. There remained as the immediate objectives a more 
productive ion source and more complete utilization of the ions. 

11.9. The factors that control the effectiveness of an ion source 
are many. Both the design of the source proper and the method 
of drawing ions from it are involved. The problems to be solved 
cannot be formulated simply and must be attacked by methods 

Electromagnetic Separation 191 

that are largely empirical. Even if security restrictions permitted 
an exposition of the innumerable forms of ion source and acceler- 
ating system that were tried, such exposition would be too 
technical to present here. 

11.10. Turning to the problem of effecting more complete 
utilization of the ions, we must consider in some detail the princi- 
ple of operation of the calutron. The calutron depends on the 
fact that singly charged ions moving in a uniform magnetic 
field perpendicular to their direction of motion are bent into 
circular paths of radius proportional to their momenta. Con- 
sidering now just a single isotope, it is apparent that the ions 

passing through the two slits (and thus passing into the large 
evacuated region in which the magnetic field is present) do not 
initially follow a single direction, but have many initial directions 
lying within a small angle, whose size depends on the width of 
the slits. Fortunately, however, since all the ions of the isotope 
in question follow curved paths of the same diameter, ions starting 
out in slightly different directions tend to meet again or almost 
meet again after completing a semicircle. It is, of course, at 
this position of reconvergence that the collector is placed. 
Naturally, the ions of another isotope (for example, ions of mass 
238 instead of 235) behave similarly, except that they follow 
circles of slightly different diameter. Samples of the two isotopes 
are caught in collectors at the two different positions of recon- 
vergence. Now the utilization of a greater fraction of the ions 
originally produced may be accomplished readily enough by 
widening the two slits referred to. But to widen the slits to any 
great extent without sacrificing sharpness of focus at the recon- 

192 Electromagnetic Separation 

vergence positions is not easy. Indeed it can be accomplished 
only by use of carefully proportioned space variations in the 
magnetic field strength. Fortunately, such variations were worked 
out successfully. 

11.11. Another problem, not so immediate but nevertheless 
recognized as important to any production plant, was that of 
more efficient use of the magnetic field. Since large electro- 
magnets are expensive both to build and to operate, it was 
natural to consider using the same magnetic field for several ion 
beams. The experimental realization of such an economical 
scheme became a major task of the laboratory. 


11.12. Although the scale of separation reached by March 1 942 
was much greater than anything that had previously been done 
with an electromagnetic mass separator, it was still very far from 
that required to produce amounts of material that would be of 
military significance. The problems that have been outlined not 
only had to be solved, but they had to be solved on a grand scale. 
The 37-inch cyclotron magnet that had been used was still 
capable of furnishing useful information, but larger equipment 
was desirable. Fortunately a very much larger magnet, intended 
for a giant cyclotron, had been under construction at Berkeley. 
This magnet, with a pole diameter of 184 inches and a pole gap 
of 72 inches, was to be the largest in existence. Work on it had 
been interrupted because of the war, but it was already suffi- 
ciently advanced so that it could be finished within a few months 
if adequate priorities were granted. Aside from the magnet itself, 
the associated building, laboratories, shops, etc., were almost 
ideal for the development of the calutron. Needless to say, work 
was resumed on the giant magnet and by the end of May 1942, 
it was ready for use. * 

* The construction of the giant cyclotron had been undertaken with 
private funds largely supplied by the Rockefeller Foundation, augmented 
by donations from the Research Corporation, the John and Mary Markel 
Foundation, and the University of California. In order to push the construe- 

Electromagnetic Separation 193 


11.13. The first experiments using the 37-inch magnet have 
been described in a previous paragraph. Later developments 
proceeded principally along these two lines: construction and 
installation of a properly engineered separation unit for the 
37-inch magnet, and design and construction of experimental 
separation units to go into the big magnet. 

11.14. Besides the gradual increase in ion beam strength and 
separation factor that resulted from a series of developments in 
the ion source and in the accelerating system, the hoped-for 
improvement in utilization of ions was achieved during the 
summer of 1 942, using the giant magnet. Further, it was possible 
to maintain more than one ion beam in the same magnetic 
separating region. Experiments on this latter problem did run 
into some difficulties, however, and it appeared that there might 
be limitations on the number of sources and receivers that could 
be put in a single unit as well as on the current that could be 
used hi each beam without spoiling the separation. 

11.15. It was evident that many separator units would be 
needed to get an amount of production of military significance. 
Therefore, consideration was given to various systems of com- 
bining groups of units in economical arrangements. A scheme 
was worked out which was later used in the production plants 
and which has proved satisfactory. 


11.16. In September 1942, both the gaseous diffusion and the 
centrifugal methods of uranium isotope separation had been 
under intensive study and for a longer period than in the case 
of the electromagnetic 'method. Both of these methods gaseous 
diffusion and centrifuge looked feasible for large-scale pro- 
duction of U-235, but both would require hundreds of stages to 
achieve large-scale separation. Neither had actually produced 

tion as fast as possible overtime work was required at additional expense. 
To cover these costs the Rockefeller Foundation made an extra appropriation. 

194 Electromagnetic Separation 

any appreciable amounts of separated U-235. No large-scale 
plant for plutonium production was under way, and the self- 
sustaining chain reaction which was to produce plutonium had 
not yet been proved attainable. But in the case of the electro- 
magnetic method, after the successful separation of milligram 
amounts, there was no question as to the scientific feasibility. 
If one unit could separate 10 mg a day, 100,000,000 units could 
separate one ton a day. The questions were of cost and time. 
Each unit was to be a complicated electromagnetic device re- 
quiring high vacuum, high voltages, and intense magnetic fields; 
and a great deal of research and development work would be 
required before complete, large-scale, units could be constructed. 
Many skilled operators would probably be needed. Altogether, at 
that time it looked very expensive, but it also looked certain and 
relatively quick. Moreover, the smallness of the units had the 
advantage that development could continue, modifications could 
be made in the course of construction or, within limits, after 
construction, and Capacity could always be expanded by building 
new units. 


11.17. On the basis of rather incomplete scientific and engi- 
neering information on all the methods and on the basis of 
equally dubious cost estimates, decisions had to be made on three 
issues: (1) whether to build an electromagnetic plant; (2) how 
big such a plant should be; (3) at what point of development the 
design should be frozen. 


11.18. On the strength of the results reported on experiments 
at Berkeley in the summer of 1942, the S-l Executive Committee, 
at a meeting at Berkeley on September 1 3-14, 1 942, recommended 
that commitments be made by the Army for an electromagnetic 
separation plant to be built at the Tennessee Valley site (Clinton 
Engineer Works). It was recommended that it should be agreed 
that commitments for this plant might be cancelled on the basis 

Electromagnetic Separation 195 

of later information. It was recommended that a pilot plant 
should be erected at the Tennessee Valley site as soon as possible. 
(However, this recommendation was subsequently withdrawn and 
such a pilot plant was never built.) The construction of a pro- 
duction plant was authorized by General Groves on November 5, 
1942, with the understanding that the design for the first units 
was to be frozen immediately. 


11.19. In describing the production of plutonium, we discussed 
the division of responsibility between the Metallurgical Project 
and the du Pont Company. The electromagnetic separation plant 
was planned and built under a somewhat different scheme of 
organization. The responsibility was divided between six major 
groups. The Radiation Laboratory at the University of California 
was responsible for research and development; the Westinghouse 
Electric and Manufacturing Company for making the mechanical 
parts, i.e., sources, receivers, pumps, tanks, etc.; the General 
Electric Company for the electrical equipment and controls; 
the Allis-Chalmers Company for the magnets; the Stone and 
Webster Engineering Company for the construction and assembly; 
and the Tennessee Eastman Company for operation. All five 
industrial concerns kept groups of their engineers at Berkeley 
so that a system of frequent informal conference and cross- 
checking was achieved. Thus the major part of the planning was 
done cooperatively in a single group, even though the details 
might be left to the home offices of the various companies. 


11.20. Strangely enough, although the theory of the self- 
sustaining chain-reacting pile is already well worked out, the 
theory of gaseous discharge, after fifty years of intensive study, 
is still inadequate for the prediction of the exact behavior of the 
ions in a calutron. The amount of U-235 collected per day, and 
the purity of the material collected, are affected by many factors, 
including: (1) the width, spacing, and shape of the collector, 

196 Electromagnetic Separation 

(2) the pressure in the .magnet space, (3) the strength and uni- 
formity of the magnetic field, (4) the shape and spacing of the 
defining slits and accelerating system, (5) the accelerating voltage, 
(6) the size and shape of the slit in the arc source from which 
the ions come, (7) the current in the arc, (8) the position of the 
arc within the arc chamber, (9) the pressure of vapor in the arc 
chamber, (10) the chemical nature of the vapor. Evidently there 
was not time for a systematic study of all possible combinations of 
variables. The development had to be largely intuitive. A variety 
of conditions had to be studied and a number of partial interpre- 
tations had to be made. Then the accumulated experience of the 
group, the "feel" of the problem, had to be translated into specific 
plans and recommendations. 


11.21. (a) The Number of Stages. As in all methods, a compro- 
mise must be made between yield and separation factor. In the 
electromagnetic system, the separation factor is much higher 
than in other systems so that the number of stages required is 
small. There was a possibility that a single stage might be suffi- 
cient. Early studies indicated that attempts to push the separation 
factor so high as to make single-stage operation feasible cut the 
yield to an impractically small figure. 

11.22. (b) Specifications. The information and experience that 
had been acquired on the variables such as those mentioned 
above had to be translated into decisions on the following princi- 
pal points before design could actually begin: (1) the size of a 
unit as determined by the radius of curvature of the ion path, 
the length of the source slit, and the arrangement of sources 
and receivers; (2) the maximum intensity of magnetic field re- 
quired; (3) whether or not to use large divergence of ion beams; 
(4) the number of ion sources and receivers per unit; (5) whether 
the source should be at high potential or at ground potential; 
(6) the number of accelerating electrodes and the maximum 
potentials to be applied to them; (7) the power requirements for 
arcs, accelerating voltages, pumps, etc.; (8) pumping require- 

Electromagnetic Separation 197 

ments; (9) number of units per pole gap; (10) number of units 
per building. 


11.23. Most of the design features for the first plant had to be 
frozen in the fall of 1942 on the basis of results obtained with 
runs made using the giant magnet at Berkeley. The plant design, 
however, called for units of a somewhat different type. While 
there was no reason to suppose that these changes would introduce 
any difference in performance, it was obviously desirable to build 
a prototype unit at Berkeley. The construction of this unit was 
approved at about the same time that the first plant units were 
ordered so that experience with it had no influence on funda- 
mental design, but it was finished and operating by April 1943, 
that is, six months before the first plant unit. Consequently, it 
was invaluable for testing and training purposes. Later, a third 
magnet was built in the big magnet building at Berkeley. All 
told, there have been six separator units available simultaneously 
for experimental or pilot plant purposes at Berkeley. Much 
auxiliary work has also been done outside the complete units. 


11.24. As we have already said, H. D. Smyth of Princeton 
became interested in electromagnetic methods of separation in 
the late summer and fall of 1941. He was particularly interested 
in devising some method of using an extended ion source and 
beam instead of one limited essentially to one dimension by a 
slit system as in the calutron mass separator. A method of actually 
achieving separation using an extended ion source was suggested 
by R. R. Wilson of Princeton. The device which resulted from 
Wilson's ideas was given the deliberately meaningless name 

11.25. The isotron is an electromagnetic mass separator using 
an extended source of ions, in contrast to the slit sources used in 
ordinary mass spectrographs. The ions from the extended source 
are first accelerated by a constant, high-intensity, electric field 

198 Electromagnetic Separation 

and are then further accelerated by a low-intensity electric field 
varying at radio frequency and in "saw tooth" manner. The 
effect of the constant electric field is to project a strong beam of 
ions down a tube with uniform kinetic energy and therefore 
with velocities inversely proportional to the square root of the 
masses of ions. The varying electric field, on the other hand, 
introduces small, periodic variations in ion velocity, and has the 
effect of causing the ions to "bunch" at a certain distance down 
the tube. (This same principle is used in the klystron high- 
frequency oscillator, where the electrons are "bunched" or 
"velocity-modulated.") The bunches of ions of different mass 
travel with different velocities and therefore become separated. 
At the position (actually on area perpendicular to the beam) 
where this occurs, an analyzer applies a transverse focussing 
electric field with a radio frequency component synchronized 
with the arrival of the bunches. The synchronization is such that 
the varying component of the transverse field strength is zero 
when the U-235 ion bunches come through and a maximum 
when the U-238 ion bunches come through. The U-235 beams 
are focussed on a collector, but the U-238 bunches are deflected. 
Thus the separation is accomplished. 

11.26. This scheme was described at the December 18, 1941 
meeting of the Uranium Committee and immediately thereafter 
was discussed more fully with Lawrence, who paid a visit to 
Princeton. The promise of the method seemed sufficient to justify 
experimental work, which was begun immediately under an 
OSRD contract and continued until February 1943. Since the 
idea involved was a novel one, there were two outstanding issues: 
(1) whether the method would work at all; (2) whether it could 
be developed for large-scale production promptly enough to 
compete with the more orthodox methods already under 

11.27. An experimental isotron was constructed and put into 
operation by the end of January 1942. Preliminary experiments 
at that time indicated that the isotopes of lithium could be 
separated by the method. The first successful collection of 

Electromagnetic Separation 199 

partially separated uranium isotopes was made in the spring 
of 1942. 

11.28. Unfortunately, progress during the summer and fall of 
1942 was not as rapid as had been hoped. Consequently, it was 
decided to close down the Princeton project in order to permit 
sending the personnel to the site where the atomic-bomb labora- 
tory was about to get under way. Before the group left Princeton 
a small experimental isotron collected several samples of partly 
separated uranium. Thus, the method worked; but its large-scale 
applicability was not fully investigated. 


11.29. In December 1941, when the whole subject of isotope 
separation was under discussion at Berkeley, the magnetron was 
suggested as a possible mass separator. In the meantime, Smyth 
of Princeton had been in contact with L. P. Smith of Cornell 
and had discovered that Smith and his students had done a 
considerable amount of work and with evidence of success 
on the separation of the isotopes of lithium by just such a method. 
This was reported to Lawrence in Washington at one of the 
December, 1941, meetings of the Uranium Committee. Lawrence 
immediately got in touch with Smith, with the result that Smith 
worked on the method at Berkeley from February 1942 to 
June 1942. J. Slepian of the Westinghouse Research Laboratory 
in East Pittsburgh came to Berkeley in the winter of 1941-1942 
at Lawrence's invitation and became interested in a modification 
of the magnetron which he called an ionic centrifuge. Slepian 
stayed at Berkeley most of the time until the fall of 1942, after 
which he returned to East Pittsburgh where he continued the 

11.30. No separation of uranium was actually attempted in 
the magnetron. Experiments with lithium with low ion currents 
showed some separation, but no consistent results were obtained 
with high ion currents. In the case of the ionic centrifuge, uranium 
samples have been collected showing appreciable separation, but 
the results have not been clear-cut or consistent. 

200 Electromagnetic Separation 


11.31. With the virtual elimination of the isotron and the 
ionic centrifuge from the development program, the calutron 
separator became the only electromagnetic method worked on 
intensively. Construction of initial units of a plant had been 
authorized and designs had been frozen for such units, but the 
whole electromagnetic program had been in existence for only 
a little more than a year and it was obvious that available 
designs were based on shrewd guesses rather than on adequate 
research. A similar situation might have occurred with the chain- 
reacting pile if unlimited amounts of uranium and graphite had 
been available before the theory had been worked out or before 
the nuclear constants had been well determined. Fortunately the 
nature of the two projects was very different, making it a less 
speculative venture to build an electromagnetic plant unit 
hastily than would have been the case for the pile. Further re- 
search and development could proceed advantageously even 
while initial units of the plant were being built and operated. 

TO JUNE 1945 


11.32. The preceding chapters show that the end of 1942 was 
a time of decision throughout the uranium project. For it was at 
that time that a self-sustaining chain reaction was first produced, 
that construction was authorized for the Hanford plutonium 
plant, the diffusion plant at Clinton, and the electromagnetic 
plant at Clinton. The diffusion plant was more flexible than the 
plutonium plant, since the diffusion plant could be broken down 
into sections and stages, built in whole or in part, to produce 
varying amounts of U-235 of varying degrees of enrichment. 
The electromagnetic plant was even more flexible, since each 
separator unit was practically independent of the other units. 
The separation process consisted of loading a charge into a unit, 

Electromagnetic Separation 201 

running the unit for a while, then stopping it and removing the 
product. To be sure, the units were built in groups, but most of 
the controls were separate for each unit. This feature made it 
possible to build the plant in steps and to start operating the first 
part even before the second was begun. It was also possible to 
change the design of subsequent units as construction proceeded; 
within limits it was possible even to replace obsolescent units in 
the early groups with new improved units. 


11.33. Construction of the first series of electromagnetic units 
at Clinton began in March of 1943 and this part of the plant was 
ready for operation in November 1943. The group at Berkeley 
continued to improve the ion sources, the receivers, and the 
auxiliary equipment, aiming always at greater ion currents. In 
fact, Berkeley reports describe no less than seventy-one different 
types of source and one hundred and fifteen different types of 
receiver, all of which reached the design stage and most of which 
were constructed and tested. As soon as the value of a given design 
change was proved, every effort was made to incorporate it in 
the designs of new units. 

11.34. Such developments as these required constant inter- 
change of information among laboratory, engineering, con- 
struction, and operating groups. Fortunately the liaison was 
excellent. The companies stationed representatives at Berkeley, 
and members of the research group at Berkeley paid frequent 
and prolonged visits to the plant at Clinton. In fact, some of the 
research men were transferred to the payroll of the Tennessee 
Eastman Company operating the plant at Clinton, and a group 
of over one hundred physicists and research engineers still kept 
on the Berkeley payroll were assigned to Clinton. Particularly 
in the early stages of operation the Berkeley men stationed at 
Clinton were invaluable as "trouble shooters" and hi instructing 
operators. A section of the plant continued to be maintained as 
a pilot unit for testing modified equipment and revised operating 
procedures, and was run jointly by the Berkeley group and by 

202 Electromagnetic Separation 

Tennessee Eastman. In addition to the British group under 
Oliphant already mentioned, there was a British group of chemists 
at Clinton under J. W. Baxter. 


11.35. Originally, the uranium salts used as sources of vapor 
for the ion-producing arcs had not been investigated with any 
very great thoroughness at Berkeley, but as the process developed, 
a good deal of work was done on these salts, and a search was 
made for a uranium compound that would be better than that 
originally used. Some valuable studies were also made on methods 
of producing the compound chosen. 

11.36. By far the most important chemical problem was the 
recovery of the processed uranium compounds from the separa- 
tion units. This recovery problem had two phases. In units of 
the first stage it was essential to recover the separated uranium 
from the receivers with maximum efficiency; whereas recovery 
of the scattered unseparated uranium from other parts of the unit 
was less important. But if higher stage units are used even the 
starting material contains a high concentration of U-235, and it 
is essential to recover all the material in the unit at the end of 
each run, i.e., material remaining in the ion source and material 
deposited on the accelerating electrodes, on the walls of the 
magnet chamber, and on the receiver walls. 


11.37. For nearly a year the electromagnetic plant was the 
only one in operation. Therefore the urge to increase its produc- 
tion rate was tremendous. It was realized that any method of 
enriching even slightly enriching the material to be fed into 
the plant would increase the production rate appreciably. For 
example, an electromagnetic unit that could produce a gram a 
day of 40 per cent pure U-235 from natural uranium could 
produce two grams a day of 80 per cent U-235 if the concentra- 
tion of U-235 in the feed material was twice the natural concen- 
tration (1.4 per cent instead of 0.7 per cent). 

Electromagnetic Separation 203 

11.38. We have already referred to the work done by P. H. 
Abelson of the Naval Research Laboratory on the separation of 
the uranium isotopes by thermal diffusion in a liquid compound 
of uranium. By the spring of 1943 Abelson had set up a pilot 
plant that accomplished appreciable separation of a considerable 
quantity of uranium compound. It was therefore proposed that 
a large-scale thermal diffusion plant should be constructed. 
Such a plant would be cheaper than any of the other large-scale 
plants, and it could be built more quickly. Its principal drawback 
\\ as its enormous consumption of steam, which made it appear 
impracticable for the whole job of separation. 

11.39. Not only was a pilot plant already in operation at the 
Naval Research Laboratory, but a second, somewhat larger 
plant was under construction at the Philadelphia Navy Yard. 
Through the cooperation of the Navy both the services of Abelson 
and the plans for a large-scale plant were made available to the 
Manhattan District. It was decided to erect the large-scale 
thermal diffusion plant at Clinton (using steam from the power 
plant constructed for the gaseous diffusion plant) and to use the 
thermal-diffusion-plant product as feed material for the electro- 
magnetic plant. 

11.40. This new thermal diffusion plant was erected in amaz- 
ingly short time during the late summer of 1944. In spite of some 
disappointments, operation of this plant has succeeded in its 
purpose of considerably increasing the production rate of the 
electromagnetic plant. It has also stimulated work on the uranium 
recovery problem. The future of this plant is uncertain. Oper- 
ation of the gaseous-diffusion plant makes it difficult to get 
enough steam to operate the thermal diffusion plant, but also 
furnishes another user for its product. 


11.41. Although the scientific and technical problems which 
confronted the Berkeley groups were probably not as varied or 
numerous as the problems encountered at Chicago and Columbia, 
they were nevertheless numerous. Thus many problems arose in 

204 Electromagnetic Separation 

the designing of the electric power and control circuits, magnetic 
fields, insulators, vacuum pumps, tanks, collectors, and sources. 
Many equipment items had to be designed from scratch and then 
mass-produced under high priority. 


11.42. The electromagnetic separation plant was in large-scale 
operation during the winter of 1944-1945, and produced U-235 
of sufficient purity for use in atomic bombs. Its operating efficiency 
is being continually improved. Research work is continuing 
although on a reduced scale. 


11.43. In the early days of the uranium project, electro- 
magnetic methods of isotope separation were rejected primarily 
because of the expected effects of space-charge. In the fall of 
1941 the question was reopened; experiments at Berkeley showed 
that space-charge effects could be largely overcome. Conse- 
quently a large-scale program for the development of electro- 
magnetic methods was undertaken. 

11.44. Of the various types of electromagnetic methods pro- 
posed, the calutron (developed at Berkeley) received principal 
attention. Two other novel methods were studied, one at Berkeley 
and one at Princeton. The calutron mass separator consists of an 
ion source from which a beam of uranium ions is drawn by an 
electric field, an accelerating system in which the ions are 
accelerated to high velocities, a magnetic field in which the ions 
travel in semicircles of radius depending on ion mass, and a 
receiving system. The principal problems of this method involved 
the ion source, accelerating system, divergence of the ion beam, 
space charge, and utilization of the magnetic field. The chief 
advantages of the calutron were large separation factor, small 
hold-up, short start-up time, and flexibility of operation. By the 
fall of 1942 sufficient progress had been made to justify authoriza- 
tion of plant construction, and a year later the first plant units 
were ready for trial at the Clinton Engineer Works in Tennessee. 

Electromagnetic Separation 205 

11.45. Research and development work on the calutron were 
carried out principally at the Radiation Laboratory of the Uni- 
versity of California, under the direction of Lawrence. Westing- 
house, General Electric, and Allis Chalmers constructed a 
majority of the parts; Stone and Webster built the plant, and 
Tennessee Eastman operated it. 

11.46. Since the calutron separation method was one of batch 
operations in a large number of largely independent units, it was 
possible to introduce important improvements even after plant 
operation had begun. 

11.47. In the summer of 1944 a thermal-diffusion separation 
plant was built at the Clinton Engineer Works to furnish en- 
riched feed material for the electromagnetic plant and thereby 
increase the production rate of this latter plant. The design of the 
thermal-diffusion plant was based on the results of research car- 
ried out at the Naval Research Laboratory and on the pilot plant 
built by the Navy Department at the Philadelphia Navy Yard. 

11.48. Although research work on the calutron was started 
later than on the centrifuge and diffusion systems, the calutron 
plant was the first to produce large amounts of the separated 
isotopes of uranium. 



12.1. The entire purpose of the work described in the preceding 
chapters was to explore the possibility of creating atomic bombs 
and to produce the concentrated fissionable materials which 
would be required in such bombs. In the present chapter, the last 
stage of the work will be described the development at Los 
Alamos of the atomic bomb itself. As in other parts of the project, 
there are two phases to be considered: the organization, and the 
scientific and technical work itself. The organization will be 
described briefly; the remainder of the chapter will be devoted 
to the scientific and technical problems. Security considerations 
prevent a discussion of many of the most important phases of this 


12.2. The project reorganization that occurred at the begin- 
ning of 1942, and the subsequent gradual transfer of the work 
from OSRD auspices to the Manhattan District have been 
described in Chapter V. It will be recalled that the responsibili- 
ties of the Metallurgical Laboratory at Chicago originally 
included a preliminary study of the physics of the atomic bomb. 
Some such studies were made in 1941; and early in 1942 
G. Breit got various laboratories (see Chapter VI, paragraph 
6.38) started on the experimental study of problems that had to 
be solved before progress could be made on bomb design. As has 
been mentioned in Chapter VI, J. R. Oppenheimer of the Uni- 
versity of California gathered a group together in the summer of 
1942 for further theoretical investigation and also undertook to 
coordinate this experimental work. This group was officially 


Work on the Atomic Bomb 207 

under the Metallurgical Laboratory but the theoretical group 
did most of its work at the University of California. By the end 
of the summer of 1 942, when General L. R. Groves took charge 
of the entire project, it was decided to expand the work consider- 
ably, and, at the earliest possible time, to set up a separate 

12.3. In the choice of a site for this atomic-bomb laboratory, 
the all-important considerations were secrecy and safety. It was 
therefore decided to establish the laboratory in an isolated loca- 
tion and to sever unnecessary connection with the outside world. 

12.4. By November 1942 a site had been chosen at Los 
Alamos, New Mexico. It was located on a mesa about 30 miles 
from Santa Fe. One asset of this site was the availability of con- 
siderable area for proving grounds, but initially the only struc- 
tures on the site consisted of a handful of buildings which once 
constituted a small boarding school. There was no laboratory, 
no library, no shop, no adequate power plant. The sole means of 
approach was a winding mountain road. That the handicaps of 
the site were overcome to a considerable degree is a tribute to 
the unstinting efforts of the scientific and military personnel. 

12.5. J. R. Oppenheimer has been director of the laboratory 
from the start. He arrived at the site in March 1943, and was 
soon joined by groups and individuals from Princeton Univer- 
sity, University of Chicago, University of California, University 
of Wisconsin, University of Minnesota, and elsewhere. With the 
vigorous support of General L. R. Groves, J. B. Conant, and 
others, Oppenheimer continued to gather around him scientists 
of recognized ability, so that the end of 1 944 found an extraordi- 
nary galaxy of scientific stars gathered on this New Mexican 
mesa. The recruiting of junior scientific personnel and technicians 
was more difficult, since for such persons the disadvantages of the 
site were not always counterbalanced by an appreciation of the 
magnitude of the goal; the use of Special Engineer Detachment 
personnel improved the situation considerably. 

12.6. Naturally, the task of assembling the necessary apparatus, 
machines, and equipment was an enormous one. Three carloads 

208 Work on the Atomic Bomb 

of apparatus from the Princeton project filled some of the most 
urgent requirements. A cyclotron from Harvard, two Van de 
Graaff generators from Wisconsin, and a Cockcroft-Walton 
high- voltage device from Illinois soon arrived. As an illustration 
of the speed with which the laboratory was set up, we may record 
that the bottom pole piece of the cyclotron magnet was not laid 
until April 14, 1943, yet the first experiment was performed in 
early July. Other apparatus was acquired in quantity; subsidiary 
laboratories were built. Today this is probably the best-equipped 
physics research laboratory in the world. 

12.7. The laboratory was financed under a contract between 
the Manhattan District and the University of California. 



12.8. In Chapter II we stated the general conditions required 
to produce a self-sustaining chain reaction. It was pointed out 
that there are four processes competing for neutrons: (1) the 
capture of neutrons by uranium which results in fission; (2) 
non-fission capture by uranium; (3) non-fission capture by 
impurities; and (4) escape of neutrons from the system. Therefore 
the condition for obtaining such a chain reaction is that process 
(1) shall produce as many new neutrons as are consumed or lost 
in all four of the processes. It was pointed out that (2) may be 
reduced by removal of U-238 or by the use of a lattice and 
moderator, that (3) may be reduced by achieving a high degree 
of chemical purity, and that (4) may be reduced (relatively) by 
increasing the size of the system. In our earlier discussions of 
chain reactions it was always taken for granted that the chain- 
reacting system must not blow up. Now we want to consider how 
to make it blow up. 

12.9. By definition, an explosion is a sudden and violent release 
of a large amount of energy in a small region. To produce an 
efficient explosion in an atomic bomb, the parts of the bomb 
must not become appreciably separated before a substantial 

Work on the Atomic Bomb 209 

fraction of the available nuclear energy has been released, since 
expansion leads to increased escape of neutrons from the system 
and thus to premature termination of the chain reaction. Stated 
differently, the efficiency of the atomic bomb will depend on the 
ratio of (a) the speed with which neutrons generated by the first 
fissions get into other nuclei and produce further fission, and (b) 
the speed with which the bomb flies apart. Using known prin- 
ciples of energy generation, temperature and pressure rise, and 
expansion of solids and vapors, it was possible to estimate the 
order of magnitude of the time interval between the beginning 
and end of the nuclear chain reaction. Almost all the technical 
difficulties of the project come from the extraordinary brevity 
of this time interval. 

12.10. In earlier chapters we stated that no self-sustaining 
chain reaction could be produced in a block of pure uranium 
metal, no matter how large, because of parasitic capture of the 
neutrons by U-238. This conclusion has been borne out by various 
theoretical calculations and also by direct experiment. For pur- 
poses of producing a non-explosive pile, the trick of using a lattice 
and a moderator suffices by reducing parasitic capture suffi- 
ciently. For purposes of producing an explosive unit, however, 
it turns out that this process is unsatisfactory on two counts. First, 
the thermal neutrons take so long (so many micro-seconds) to 
act that only a feeble explosion would result. Second, a pile is 
ordinarily far too big to be transported. It is therefore necessary 
to cut down parasitic capture by removing the greater part of the 
U-238 or to use plutonium. 

12.11. Naturally, these general principles and others had 
been well established before the Los Alamos project was set up. 


12.12. The calculation of the critical size of a chain-reacting 
unit is a problem that has already been discussed in connection 
with piles. Although the calculation is simpler for a homogeneous 
metal unit than for a lattice, inaccuracies remained in the course 
of the early work, both because of lack of accurate knowledge of 

210 Work on the Atomic Bomb 

constants and because of mathematical difficulties. For example, 
the scattering, fission, and absorption cross sections of the nuclei 
involved all vary with neutron velocity. The details of such varia- 
tion were not known experimentally and were difficult to take 
into account in making calculations. By the spring of 1 943 several 
estimates of critical size had been made using various methods of 
calculation and using the best available nuclear constants, but 
the limits of error remained large. 


12.13. In a uranium-graphite chain-reacting pile the critical 
size may be considerably reduced by surrounding the pile with 
a layer of graphite, since such an envelope "reflects" many neu- 
trons back into the pile. A similar envelope can be used to reduce 
the critical size of the bomb, but here the envelope has an addi- 
tional role: its very inertia delays the expansion of the reacting 
material. For this reason such an envelope is often called a 
tamper. Use of a tamper clearly makes for a longer lasting, more 
energetic, and more efficient explosion. The most effective tamper 
is the one having the highest density; high tensile strength turns 
out to be unimportant. It is a fortunate coincidence that mate- 
rials of high density are also excellent as reflectors of neutrons. 


12.14. As has already been remarked, the bomb tends to fly 
to bits as the reaction proceeds and this tends to stop the reaction. 
To calculate how much the bomb has to expand before the 
reaction stops is relatively simple. The calculation of how long 
this expansion takes and how far the reaction goes in that time 
is much more difficult. 

12.15. While the effect of a tamper is to increase the efficiency 
both by reflecting neutrons and by delaying the expansion of 
the bomb, the effect on the efficiency is not as great as on the 
critical mass. The reason for this is that the process of reflection 
is relatively time-consuming and may not occur extensively 
before the chain reaction is terminated. 

Work on the Atomic Bomb 21 1 


12.16. As stated in Chapter II, it is impossible to prevent a 
chain reaction from occurring when the size exceeds the critical 
size. For there are always enough neutrons (from cosmic rays, 
from spontaneous fission reactions, or from alpha-particle-in- 
duced reactions in impurities) to initiate the chain. Thus until 
detonation is desired, the bomb must consist of a number of 
separate pieces each one of which is below the critical size either 
by reason of small size or unfavorable shape. To produce 
detonation, the parts of the bomb must be brought together 
rapidly. In the course of this assembly process the chain reaction 
is likely to start because of the presence of stray neutrons 
before the bomb has reached its most compact (most reactive) 
form. Thereupon the explosion tends to prevent the bomb from 
reaching that most compact form. Thus it may turn out that 
the explosion is so inefficient as to be relatively useless. The 
problem, therefore, is two-fold: (1) to reduce the time of assembly 
to a minimum; and (2) to reduce the number of stray (pre- 
detonation) neutrons to a minimum. 

12.17. Some consideration was given to the danger of pro- 
ducing a "dud" or a detonation so inefficient that even the bomb 
itself would not be completely destroyed. This would, of course, 
present the enemy with a supply of highly valuable material. 


12.18. In Chapters II and IV it was pointed out that the 
amount of energy released was not the sole criterion of the value 
of a bomb. There was no assurance that one uranium bomb 
releasing energy equal to the energy released by 20,000 tons of 
TNT would be as effective in producing military destruction as, 
say, 10,000 two-ton bombs. In fact, there were good reasons to 
believe that the destructive effect per calorie released decreases 
as the total amount of energy released increases. On the other 
hand, in atomic bombs the total amount of energy released per 
kilogram of fissionable material (i.e., the efficiency of energy 

212 Work on the Atomic Bomb 

release) increases with the size of the bomb. Thus the optimum 
size of the atomic bomb was not easily determined. A tactical 
aspect that complicates the matter further is the advantage of 
simultaneous destruction of a large area of enemy territory. In 
a complete appraisal of the effectiveness of an atomic bomb, 
attention must also be given to effects on morale.* The bomb 
is detonated in combat at such a height above the ground as to 
give the maximum blast effect against structures, and to dis- 
seminate the radioactive products as a cloud. On account of the 
height of the explosion practically all the radioactive products 
are carried upward in the ascending column of hot air and dis- 
persed harmlessly over a wide area. Even in the New Mexico 
test, where the height of explosion was necessarily low, only a 
very small fraction of the radioactivity was deposited immediately 
below the bomb. 


12.19. Since estimates had been made of the speed that would 
bring together subcritical masses of U-235 rapidly enough to 
avoid predetonation, a good deal of thought had been given to 
practical methods of doing this. The obvious method of very 
rapidly assembling an atomic bomb was to shoot one part as a 
projectile in a gun against a second part as a target. The projectile 
mass, projectile speed, and gun caliber required were not far 
from the range of standard ordnance practice, but novel problems 
were introduced by the importance of achieving sudden and 
perfect contact between projectile and target, by the use of 
tampers, and by the requirement of portability. None of these 
technical problems had been studied to any appreciable extent 
prior to the establishment of the Los Alamos laboratory. 

12.20. It had also been realized that schemes probably might 
be devised whereby neutron absorbers could be incorporated in 
the bomb in such a way that they would be rendered less effective 
by the initial stages of the chain reactions. Thus the tendency for 

* The rest of this paragraph is from a War Department release subsequent 
to the first use of atomic bombs against Japan. 

Work on the Atomic Bomb 213 

the bomb to detonate prematurely and inefficiently would be 
minimized. Such devices for increasing the efficiency of the bomb 
are called auto-catalytic. 


12.21. In April 1943 the available information of interest in 
connection with the design of atomic bombs was preliminary 
and inaccurate. Further and extensive theoretical work on critical 
size, efficiency, effect of tamper, method of detonation, and 
effectiveness was urgently needed. Measurements of the nuclear 
constants of U-235, plutonium, and tamper material had to be 
extended and improved. In the cases of U-235 and plutonium, 
tentative measurements had to be made using only minute 
quantities until larger quantities became available. 

12.22. Besides these problems in theoretical and experimental 
physics, there was a host of chemical, metallurgical, and technical 
problems that had hardly been touched. Examples were the 
purification and fabrication of U-235 and plutonium, and the 
fabrication of the tamper. Finally, there were problems of instan- 
taneous assembly of the bomb that were staggering in their 



12.23. For administrative purposes the scientific staff at Los 
Alamos was arranged in seven divisions, which have been re- 
arranged at various times. During the spring of 1 945 the divisions 
were: Theoretical Physics Division under H. Bethe, Experi- 
mental Nuclear Physics Division under R. R. Wilson, Chemistry 
and Metallurgy Division under J. W. Kennedy and C. S. Smith, 
Ordnance Division under Capt. W. S. Parsons (USN), Explosives 
Division under G. B. Kistiakowsky, Bomb Physics Division under 
R. F. Bacher, and an Advanced Development Division under 
E. Fermi. All the divisions reported to J. R. Oppenheimer, 
Director of the Los Alamos Laboratory who has been assisted in 

214 Work on the Atomic Bomb 

coordinating the research by S. K. Allison since December 1944. 
J. Chad wick of England and N. Bohr of Denmark spent a great 
deal of time at Los Alamos and gave invaluable advice. Chadwick 
was the head of a British delegation which contributed materially 
to the success of the laboratory. For security reasons, most of the 
work of the laboratory can be described only in part. 


12.24. There were two considerations that gave unusual 
importance to the work of the Theoretical Physics Division under 
H. Bethe. The first of these was the necessity for effecting simul- 
taneous development of everything from the fundamental mate- 
rials to the method of putting them to use all despite the virtual 
unavailability of the principal materials (U-235 and plutonium) 
and the complete novelty of the processes. The second considera- 
tion was the impossibility of producing (as for experimental 
purposes) a "small-scale" atomic explosion by making use of 
only a small amount of fissionable material. (No explosion occurs 
at all unless the mass of the fissionable material exceeds the 
critical mass.) Thus it was necessary to proceed from data ob- 
tained in experiments on infinitesimal quantities of materials 
and to combine it with the available theories as accurately as 
possible in order to make estimates as to what would happen in 
the bomb. Only in this way was it possible to make sensible plans 
for the other parts of the project, and to make decisions on design 
and construction without waiting for elaborate experiments on 
large quantities of material. To take a few examples, theoretical 
work was required in making rough determinations of the dimen- 
sions of the gun, in guiding the metallurgists in the choice of 
tamper materials, and in determining the influence of the purity 
of the fissionable material on the efficiency of the bomb. 

12.25. The determination of the critical size of the bomb was 
one of the main problems of the Theoretical Physics Division. In 
the course of time, several improvements were made in the 
theoretical approach whereby it was possible to take account of 
practically all the complex phenomena involved. It was at first 

Work on the Atomic Bomb 215 

considered that the diffusion of neutrons was similar to the diffu- 
sion of heat, but this naive analogy had to be forsaken. In the 
early theoretical work the assumptions were made that the 
neutrons all had the same velocity and all were scattered iso- 
tropically. A method was thus developed which permitted cal- 
culation of the critical size for various shapes of the fissionable 
material provided that the mean free path of the neutrons was 
the same in the tamper material as in the fissionable material. 
This method was later improved first by taking account of the 
angular dependence of the scattering and secondly by allowing 
for difference in mean free path in core and tamper materials. 
Still later, means were found of taking into account the effects 
of the distribution in velocity of the neutrons, the variations of 
cross sections with velocity, and inelastic scattering in the core 
and tamper materials. Thus it became possible to compute 
critical sizes assuming almost any kind of tamper material. 

12.26. The rate at which the neutron density decreases in 
bomb models which are smaller than the critical size can be cal- 
culated, and all the variables mentioned above can be taken into 
account. The rate of approach to the critical condition as the 
projectile part of the bomb moves toward the target part of the 
bomb has been studied by theoretical methods. Furthermore, 
the best distribution of fissionable material in projectile and target 
was determined by theoretical studies. 

12.27. Techniques were developed for dealing with set-ups in 
which the number of neutrons is so small that a careful statistical 
analysis must be made of the effects of the neutrons. The most 
important problem in this connection was the determination of 
the probability that, when a bomb is larger than critical size, 
a stray neutron will start a continuing chain reaction. A related 
problem was the determination of the magnitude of the fluctua- 
tions in neutron density in a bomb whose size is close to the critical 
size. By the summer of 1945 many such calculations had been 
checked by experiments. 

12.28. A great deal of theoretical work was done on the equa- 
tion of state of matter at the high temperatures and pressures to 

216 Work on the Atomic Bomb 

be expected in the exploding atomic bombs. The expansion of 
the various constituent parts of the bomb during and after the 
moment of chain reaction has been calculated. The effects of 
radiation have been investigated in considerable detail. 

12.29. Having calculated the energy that is released in the 
explosion of an atomic bomb, one naturally wants to estimate 
the military damage that will be produced. This involves analysis 
of the shock waves in air and in earth, the determination of the 
effectiveness of a detonation beneath the surface of the ocean, etc. 

12.30. In addition to all the work mentioned above, a con- 
siderable amount of work was done in evaluating preliminary 
experiments. Thus an analysis was made of the back-scattering 
of neutrons by the various tamper materials proposed. An 
analysis was also made of the results of experiments on the 
multiplication of neutrons in subcritical amounts of fissionable 


12.31. The experiments performed by the Experimental 
Nuclear Physics group at Los Alamos were of two kinds: "differ- 
ential" experiments as for determining the cross section for fission 
of a specific isotope by neutrons of a specific velocity, and "in- 
tegral" experiments as for determining the average scattering of 
fission neutrons from an actual tamper. 

12.32. Many nuclear constants had already been determined 
at the University of Chicago Metallurgical Laboratory and else- 
where, but a number of important constants were still undeter- 
mined especially those involving high neutron velocities. Some 
of the outstanding questions were the following: 

1. What are the fission cross sections of U-234, U-235, U-238, 
Pu-239, etc.? How do they vary with neutron velocity? 

2. What are the elastic scattering cross sections for the same 
nuclei (also for nuclei of tamper materials)? How do they vary 
with neutron velocity? 

3. What are the inelastic cross sections for the nuclei referred 
to above? 

Work on the Atomic Bomb 217 

4. Wh'at are the absorption cross sections for processes other 
than fission? 

5. How many neutrons are emitted per fission in the case of 
each of the nuclei referred to above? 

6. What is the full explanation of the fact that the number of 
neutrons emitted per fission is not a whole number? 

7. What is the initial energy of the neutrons produced by 

8. Does the number or energy of such neutrons vary with the 
speed of the incident neutrons? 

9. Are fission neutrons emitted immediately? 

10. WTiat is the probability of spontaneous fission of the various 
fissionable nuclei? 

12.33. In addition to attempting to find the answers to these 
questions the Los Alamos Experimental Nuclear Physics Division 
investigated many problems of great scientific interest which 
were expected to play a role in their final device. Whether or not 
this turned out to be the case, the store of knowledge thus accumu- 
lated by the Division forms an integral and invaluable part of all 
thinking on nuclear problems. 

12.34. Experimental Methods. The earlier chapters contain little 
or no discussion of experimental techniques except those for the 
observing of fast (charged) particles (See Appendix 1.). To obtain 
answers to the ten questions posed above, we should like to be 
able to: 

(1) determine the number of neutrons of any given energy; 

(2) produce neutrons of any desired energy; 

(3) determine the angles of deflection of scattered neutrons; 

(4) determine the number of fissions occurring; 

(5) detect other consequences of neutron absorption, e.g., 
artificial radioactivity. 

We shall indicate briefly how such observations are made. 

12.35. Detection of Neutrons. There are three ways in which 
neutrons can be detected: by the ionization produced by light 
atomic nuclei driven forward at high speeds by elastic collisions 

218 Work on the Atomic Bomb 

with neutrons, by the radioactive disintegration of unstable 
nuclei formed by the absorption of neutrons, and by fission 
resulting from neutron absorption. All three processes lead to 
the production of ions and the resulting ionization may be 
detected using electroscopes, ionization chambers, Geiger-Miiller 
counters, Wilson cloud chambers, tracks in photographic emul- 
sion, etc. 

12.36. While the mere detection of neutrons is not difficult, 
the measurement of the neutron velocities is decidedly more so. 
The Wilson cloud chamber method and the photographic 
emulsion method give the most direct results but are tedious to 
apply. More often various combinations of selective absorbers 
are used. Thus, for example, if a foil known to absorb neutrons 
of only one particular range of energies is inserted in the path of 
the neutrons and is then removed, its degree of radioactivity is 
presumably proportional to the number of neutrons in the 
particular energy range concerned. Another scheme is to study 
the induced radioactivity known to be produced only by neutrons 
whose energy lies above a certain threshold. 

12.37. One elegant scheme for studying the effects of neutrons 
of a single, arbitrarily-selected velocity is the "time of flight" 
method. In this method a neutron source is modulated, i.e., the 
source is made to emit neutrons in short "bursts" or "pulses." 
In each pulse there are a great many neutrons of a very wide 
range of velocities. The target material and the detector are 
situated a considerable distance from the source (several feet 
or yards from it). The detector is "modulated" also, and with 
the same periodicity. The timing or phasing is made such that 
the detector is responsive only for a short interval beginning a 
certain time after the pulse of neutrons leaves the source. Thus 
any effects recorded by the detector (e.g., fissions in a layer of 
uranium deposited on an inner surface of an ionization chamber) 
are the result only of neutrons that arrive just at the moment of 
responsivity and therefore have travelled from the source in a 
certain time interval. In other words, the measured effects are 
due only to the neutrons having the appropriate velocity. 

Work on the Atomic Bomb 219 

12.38. Production of Neutrons. All neutrons are produced as the 
result of nuclear reactions, and their initial speed depends on the 
energy balance of the particular reaction. If the reaction is 
endothermic, that is, if the total mass of the resultant particles is 
greater than that of the initial particles, the reaction does not 
occur unless the bombarding particle has more than the "thresh- 
old"' kinetic energy. At higher bombarding energies the kinetic 
energy- of the resulting particles, specifically of the neutrons, goes 
up with the increase of kinetic energy of the bombarding particle 
above the threshold value. Thus the Li 7 (p, n)Be 7 reaction absorbs 
1.6 Mev energy since the product particles are heavier than the 
initial particles. Any further energy of the incident protons goes 
into kinetic energy of the products so that the maximum speed 
of the neutrons produced goes up with the speed of the incident 
protons. However, to get neutrons of a narrow range of speed, a 
thin target must be used, the neutrons must all come off at the 
same angle, and the protons must all strike the target with the 
same speed. 

12.39. Although the same energy and momentum conserva- 
tion laws apply to exothermic nuclear reactions, the energy 
release is usually large compared to the kinetic energy of the 
bombarding particles and therefore essentially determines the 
neutron speed. Often there are several ranges of speed from 
the same reaction. There are some reactions that produce very 
high energy neutrons (nearly 15 Mev). 

12.40. Since there is a limited number of nuclear reactions 
usable for neutron sources, there are only certain ranges of 
neutron speeds that can be produced originally. There is no 
difficulty about slowing down neutrons, but it is impossible to 
slow them down uniformly, that is, without spreading out the 
velocity distribution. The most effective slowing-down scheme is 
the use of a moderator, as in the graphite pile; in fact, the pile 
itself is an excellent source of thermal (i.e., very low speed) or 
nearly thermal neutrons. 

12.41. Determination of Angles of Deflection. The difficulties in 
measuring the angles of deflection of neutrons are largely of 

220 Work on the Atomic Bomb 

intensity and interpretation. The number of neutrons scattered 
in a particular direction may be relatively small, and the "scat- 
tered" neutrons nearly always include many strays not coming 
from the intended target. 

12.42. Determination of Number of Fissions. The determination 
of the number of fissions which are produced by neutrons or 
occur spontaneously is relatively simple. lonization chambers, 
counter tubes, and many other types of detectors can be used. 

12.43. Detection of Products of Capture of Neutrons. Often it is 
desirable to find in detail what has happened to neutrons that 
are absorbed but have not produced fission, e.g., resonance or 
"radiative" capture of neutrons by U-238 to form U-239 which 
leads to the production of plutonium. Such studies usually 
involve a combination of microchemical separations and radio- 
activity analyses. 

12.44. Some Experiments on Nuclear Constants. By the time that 
the Los Alamos laboratory had been established, a large amount 
of work had been done on the effects of slow neutrons on the 
materials then available. For example, the thermal-neutron 
fission cross section of natural uranium had been evaluated, and 
similarly for the separated isotopes of uranium and for plutonium. 
Some data on high-speed-neutron fission cross sections had been 
published, and additional information was available in project 
laboratories. To extend and improve such data, Los Alamos per- 
fected the use of the Van de Graaff generator for the Li 7 (p, n)Be 7 
reaction, so as to produce neutrons of any desired energy lying 
in the range from 3,000 electron volts to two million electron 
volts. Success was also achieved in modulating the cyclotron 
beam and developing the neutron time-of-flight method to 
produce effects of many speed intervals at once. Special methods 
were devised for filling in the gaps in neutron energy range. 
Particularly important was the refinement of measurement made 
possible as greater quantities of U-235, U-238 and plutonium 
began to be received. On the whole, the value of the cross section 
for fission as a function of neutron energy from practically zero 
electron volts to three million electron volts is now fairly well 
known for these materials. 

Work on the Atomic Bomb 221 

12.45. Some Integral Experiments. Two "integral experiments" 
(experiments on assembled or integrated systems comprising 
fissionable material, reflector, and perhaps moderator also) may 
be described. In the first of these integral experiments a chain- 
reacting system was constructed which included a relatively 
large amount of U-235 in liquid solution. It was designed to 
operate at a very low power level, and it had no cooling system. 
Its purpose was to provide verification of the effects predicted 
for reacting systems containing enriched U-235. The results were 
very nearly as expected. 

12.46. The second integral experiment was carried out on a 
pile containing a mixture of uranium and a hydrogenous moder- 
ator. In this first form, the pile was thus a slow-neutron chain- 
reacting pile. The pile was then rebuilt using less hydrogen. In 
this version of the pile, fast-neutron fission became important. 
The pile was rebuilt several more times, less hydrogen being 
used each time. By such a series of reconstructions, the reaction 
character was successively altered, so that thermal neutron fission 
became less and less important while fast neutron fission became 
more and more important approaching the conditions to be 
found in the bomb. 

12.47. Summary of Results on Nuclear Physics. The nuclear con- 
stants of U-235, U-238, and plutonium have been measured with 
a reasonable degree of accuracy over the range of neutron energies 
from thermal to three million electron volts. In other words, 
questions 1, 2, 3, 4, and 5 of the ten questions posed at the 
beginning of this section have been answered. The fission spectrum 
(question 7) for U-235 and Pu-239 is reasonably well known. 
Spontaneous fission (question 10) has been studied for several 
types of nuclei. Preliminary results on questions 6, 8, and 9, 
involving details of the fission process, have been obtained. 


12.48. The Chemistry and Metallurgy Division of the Los 
Alamos Laboratory was under the joint direction of J. W. 
Kennedy and C. S. Smith. It was responsible for final purification 
of the enriched fissionable materials, for fabrication of the bomb 

222 Work on the Atomic Bomb 

core, tamper, etc., and for various other matters. In all this 
division's work on enriched fissionable materials especial care 
had to be taken not to lose any appreciable amounts of the ma- 
terials which are worth much more than gold. Thus the pro- 
cedures already well-established at Chicago and elsewhere for 
purifying and fabricating natural uranium were often not satis- 
factory for handling highly-enriched samples of U-235. 


12.49. The above account of the work of the Theoretical 
Physics, Experimental Nuclear Physics, and Chemistry and 
Metallurgy Divisions is very incomplete because important 
aspects of this work cannot be discussed for reasons of security. 
For the same reasons none of the work of the Ordnance, Explo- 
sives, and Bomb Physics Divisions can be discussed at all. 


12.50. In the spring of 1943 an entirely new laboratory was 
established at Los Alamos, New Mexico, under J. R. Oppen- 
heimer for the purpose of investigating the design and construc- 
tion of the atomic bomb, from the stage of receipt of U-235 or 
plutonium to the stage of use of the bomb. The new laboratory 
improved the theoretical treatment of design and performance 
problems, refined and extended the measurements of the nuclear 
constants involved, developed methods of purifying the materials 
to be used, and, finally, designed and constructed operable atomic 



13.1. As the result of the labors of the Manhattan District 
organization in Washington and in Tennessee, of the scientific 
groups at Berkeley, Chicago, Columbia, Los Alamos, and else- 
where, of the industrial groups at Clinton, Hanford, and many 
other places, the end of June 1945 finds us expecting from day to 
day to hear of the explosion of the first atomic bomb devised by 
man. All the problems are believed to have been solved at least 
well enough to make a bomb practicable. A sustained neutron 
chain reaction resulting from nuclear fission has been demon- 
strated; the conditions necessary to cause such a reaction to 
occur explosively have been established and can be achieved; 
production plants of several different types are in operation, 
building up a stock pile of the explosive material. Although we 
do not know when the first explosion will occur nor how effective 
it will be, announcement of its occurrence will precede the pub- 
lication of this report. Even if the first attempt is relatively 
ineffective, there is little doubt that later efforts will be highly 
effective; the devastation from a single bomb is expected to be 
comparable to that of a major air raid by usual methods. 

13.2. A weapon has been developed that is potentially destruc- 
tive beyond the wildest nightmares of the imagination; a w r eapon 
so ideally suited to sudden unannounced attack that a country's 
major cities might be destroyed overnight by an ostensibly 
friendly power. This weapon has been created not by the devilish 
inspiration of some warped genius but by the arduous labor of 
thousands of normal men and women working for the safety of 
their country. Many of the principles that have been used were 
well known to the international scientific world in 1940. To 
develop the necessary industrial processes from these principles 


224 General Summary 

has been costly in time, effort, and money, but the processes 
which we selected for serious effort have worked and several 
that we have not chosen could probably be made to work. We 
have an initial advantage in time because, so far as we know, 
other countries have not been able to carry out parallel develop- 
ments during the war period. We also have a general advantage 
in scientific and particularly in industrial strength, but such an 
advantage can easily be thrown away. 

13.3. Before the surrender of Germany there was always a 
chance that German scientists and engineers might be developing 
atomic bombs which would be sufficiently effective to alter the 
course of the war. There was therefore no choice but to work on 
them in this country. Initially many scientists could and did hope 
that some principle would emerge which would prove that 
atomic bombs were inherently impossible. This hope has faded 
gradually; fortunately in the same period the magnitude of the 
necessary industrial effort has been demonstrated so that the fear 
of German success weakened even before the end came. By the 
same token, most of us are certain that the Japanese cannot 
develop and use this weapon effectively. 


13.4. As to the future, one may guess that technical develop- 
ments will take place along two lines. From the military point 
of view it is reasonably certain that there will be improvements 
both in the processes of producing fissionable material and in its 
use. It is conceivable that totally different methods may be dis- 
covered for converting matter into energy since it is to be remem- 
bered that the energy released in uranium fission corresponds to 
the utilization of only about one-tenth of one per cent of its mass. 
Should a scheme be devised for converting to energy even as 
much as a few per cent of the matter of some common material, 
civilization would have the means to commit suicide at will. 

13.5. The possible uses of nuclear energy are not all destruc- 
tive, and the second direction in which technical development can 
be expected is along the paths of peace. In the fall of 1 944 General 

General Summary 225 

Groves appointed a committee to look into these possibilities as 
well as those of military significance. This committee (Dr. R. C. 
Tolman, chairman; Rear Admiral E. W. Mills (USN) with 
Captain T. A. Solberg (USN) as deputy, Dr. W. K. Lewis, and 
Dr. H. D. Smyth) received a multitude of suggestions from men 
on the various projects, principally along the lines of the use of 
nuclear energy for power and the use of radioactive by-products 
for scientific, medical, and industrial purposes. While there was 
general agreement that a great industry might eventually arise, 
comparable, perhaps, with the electronics industry, there was 
disagreement as to how rapidly such an industry would grow; the 
consensus was that the growth would be slow over a period of 
many years. At least there is no immediate prospect of running 
cars with nuclear power or lighting houses with radioactive 
lamps although there is a good probability that nuclear power 
for special purposes could be developed within ten years and that 
plentiful supplies of radioactive materials can have a profound 
effect on scientific research and perhaps on the treatment of 
certain diseases in a similar period. 


13.6. During the war the effort has been to achieve the maxi- 
mum military results. It has been apparent for some time that 
some sort of government control and support in the field of 
nuclear energy must continue after the war. Many of the men 
associated with the project have recognized this fact and have 
come forward with various proposals, some of which were con- 
sidered by the Tolman Committee, although it was only a 
temporary advisory committee reporting to General Groves. An 
interim committee at a high level is now engaged in formulating 
plans for a continuing organization. This committee is also 
discussing matters of general policy about which many of the 
more thoughtful men on the project have been deeply concerned 
since the work was begun and especially since success became 
more and more probable. 

226 General Summary 


13.7. We find ourselves with an explosive which is far from 
completely perfected. Yet the future possibilities of such explosives 
are appalling, and their effects on future wars and international 
affairs are of fundamental importance. Here is a new tool for 
mankind, a tool of unimaginable destructive power. Its develop- 
ment raises many questions that must be answered in the near 

13.8. Because of the restrictions of military security there has 
been no chance for the Congress or the people to debate such 
questions. They have been seriously considered by all concerned 
and vigorously debated among the scientists, and the conclusions 
reached have been passed along to the highest authorities. These 
questions are not technical questions; they are political and social 
questions, and the answers given to them may affect all mankind 
for generations. In thinking about them the men on the project 
have been thinking as citizens of the United States vitally inter- 
ested in the welfare of the human race. It has been their duty 
and that of the responsible high government officials who were 
informed to look beyond the limits of the present war and its 
weapons to the ultimate implications of these discoveries. This 
was a heavy responsibility. In a free country like ours, such 
questions should be debated by the people and decisions must 
be made by the people through their representatives. This is one 
reason for the release of this report. It is a semi-technical report 
which it is hoped men of science in this country can use to help 
their fellow citizens in reaching wise decisions. The people of 
the country must be informed if they are to discharge their 
responsibilities wisely. 


In Chapter I we pointed out the importance of ionization in the 
study of radioactivity and mentioned the electroscope. In this 
appendix we shall mention one method of historical importance 
comparable with the electroscope but no longer used, and then 
we shall review the various methods now in use for observing 
alpha particles, beta particles (or positrons), gamma rays, and 
neutrons, or their effects. 


The closest approach that can be made to "seeing" an atom is 
to see the bright flash of light that an alpha particle or high-speed 
proton makes when it strikes a fluorescent screen. All that is 
required is a piece of glass covered with zinc sulphide, a low- 
power microscope, a dark room, a well-rested eye, and a source 
of alpha particles. Most of Rutherford's famous experiments, 
including that mentioned in paragraph 1.17, involved "counting" 
scintillations but the method is tedious and, as far as the author 
knows, has been entirely superseded by electrical methods. 


When a high-speed charged particle like an alpha particle or a 
high-speed electron passes through matter, it disrupts the mole- 
cules that it strikes by reason of the electrical forces between the 
charged particle and the electrons in the molecule. If the material 
is gaseous, the resultant fragments or ions may move apart and, 
if there is an electric field present, the electrons knocked out of 
the molecules move in one direction and the residual positive 
ions in another direction. A beta particle with a million electron 
volts energy will produce some 18,000 ionized atoms before it is 


228 Appendix 1 

stopped completely since on the average it uses up about 60 volts 
energy in each ionizing collision. Since each ionization process 
gives both a positive and a negative ion, there is a total of 36,000 
charges set free by one high-speed electron, but since each charge 
is only 1.6 X 10~ 19 coulomb, the total is only about 6 X 10~ 15 
coulomb and is still very minute. The best galvanometer can be 
made to measure a charge of about 10~ 10 coulomb. It is posssible 
to push the sensitivity of an electrometer to about 10~ 16 coulomb, 
but the electrometer is a very inconvenient instrument to use. 

An alpha particle produces amounts of ionization comparable 
with the beta particle. It is stopped more rapidly, but it produces 
more ions per unit of path. A gamma ray is much less efficient 
as an ionizer since the process is quite different. It does occa- 
sionally set free an electron from a molecule by Compton scatter- 
ing or the photoelectric effect, and this secondary electron has 
enough energy to produce ionization. A neutron, as we have 
already mentioned in the text, produces ionization only indirectly 
by giving high velocity to a nucleus by elastic collision, or by 
disrupting a nucleus with resultant ionization by the fragments. 

If we are to detect the ionizing effects of these particles, we 
must evidently use the resultant effect of a great many particles 
or have very sensitive means of measuring electric currents. 


Essentially the electroscope determines to what degree the air 
immediately around it has become conducting as the result of 
the ions produced in it. 

The simplest form of electroscope is a strip of gold leaf a few 
centimeters long, suspended by a hinge from a vertical insulated 
rod. If the rod is charged, the gold leaf also takes up the same 
charge and stands out at an angle as a result of the repulsion of 
like charges. As the charge leaks away, the leaf gradually swings 
down against the rod, and the rate at which it moves is a measure 
of the conductivity of the air surrounding it. 

A more rugged form of electroscope was devised by C. C. 
Lauritsen, who substituted a quartz fiber for the gold leaf and 

Observing Fast Particles 229 

used the elasticity of the fiber as the restoring force instead of 
gravity. The fiber is made conducting by a thin coating of metal. 
Again the instrument is charged, and the fiber, after initial 
deflection, gradually comes back to its uncharged position. The 
position of the fiber is read in a low-power microscope. These 
instruments can be made portable and rugged and fairly sensi- 
tive. They are the standard field instrument for testing the level 
of gamma radiation, particularly as a safeguard against dangerous 


An ionization chamber measures the total number of ions 
produced directly in it. It usually consists of two plane electrodes 
between which there is a strong enough electric field to draw all 
the ions to the electrodes before they recombine but not strong 
enough to produce secondary ions as in the instruments we shall 
describe presently. 

By careful design and the use of sensitive amplifiers an ioniza- 
tion chamber can measure a number of ions as low as that pro- 
duced by a single alpha particle, or it can be used much like an 
electroscope to measure the total amount of ionizing radiation 
present instantaneously, or it can be arranged to give the total 
amount of ionization that has occurred over a period of time. 


While ionization chambers can be made which will respond 
to single alpha particles, it is far more convenient to use a self- 
amplifying device, that is, to make the ions originally produced 
make other ions in the same region so that the amplifier circuits 
need not be so sensitive. 

In a proportional counter one of the electrodes is a fine wire 
along the axis of the second electrode, which is a hollow cylinder. 
The effect of the wire is to give strong electric field strengths close 
to it even for relatively small potential differences between it 
and the other electrode. This strong field quickly accelerates the 

230 Appendix 1 

primary ions formed by the alpha or beta particle or photon, and 
these accelerated primary ions (particularly the electrons) in 
turn form secondary ions in the gas with which the counter is 
filled so that the total pulse of current is much increased. 

It is possible to design and operate such counters in such a way 
that the total number of ions formed is proportional to the num- 
ber of primary ions formed. Thus after amplification a current 
pulse can be seen on an oscilloscope, the height of which will 
indicate how effective an ionizer the initial particle was. It is 
quite easy to distinguish in this way between alpha particles and 
beta particles and photons, and the circuits can be arranged to 
count only the pulses of greater than a chosen magnitude. Thus 
a proportional counter can count alpha particles against a back- 
ground of betas or can even count only the alpha particles having 
more than a certain energy. 


If the voltage on a proportional counter is raised, there comes 
a point when the primary ions from a single alpha particle, beta 
particle, or photon will set off a discharge through the whole 
counter, not merely multiply the number of primary ions in the 
region where they are produced. This is a trigger action and the 
current is independent of the number of ions produced ; further- 
more, the current would continue indefinitely if no steps were 
taken to quench it. Quenching can be achieved entirely by 
arranging the external circuits so that the voltage drops as soon 
as current passes or by using a mixture of gases in the counter 
which "poison" the electrode surface as soon as the discharge 
passes and temporarily prevent the further emission of electrons, 
or by combining both methods. 

The Geiger-Muller counter was developed before the propor- 
tional counter and remains the most sensitive instrument for 
detecting ionizing radiation, but all it does is "count" any ionizing 
radiation that passes through it whether it be an alpha particle, 
proton, electron, or photon. 

Observing Fast Particles 231 


It is one thing to describe the principles of various ionization 
chambers, counters, and the like; quite another to construct and 
operate them successfully. 

First of all, the walls of the counter chamber must allow the 
particles to enter the counter. For gamma rays this is a minor 
problem, but for relatively low-speed electrons or positrons or for 
alpha particles the walls of the counter must be very thin or 
there must be thin windows. 

Then there are great variations in the details of the counter 
itself, spacing and size of electrodes, nature of the gas filling the 
chamber, its pressure, and so on. 

Finally, the interpretation of the resultant data is a tricky 
business. The absorption of the counter walls and of any external 
absorbers must be taken into account; the geometry of the 
counter with relation to the source must be estimated to translate 
observed counts into actual number of nuclear events; last but 
not always least, statistical fluctuations must be considered since 
all nuclear reactions are governed by probability laws. 


There is one method of observing nuclear particles that depends 
directly on ionization but is not an electrical method. It uses the 
fact that supersaturated vapor will condense more readily on 
ions than on neutral molecules. If air saturated with water vapor 
is cooled by expansion just after an alpha particle has passed 
through it, tiny drops of water condense on the ions formed by 
the alpha particle and will reflect a bright light strongly enough 
to be seen or photographed so that the actual path of the alpha 
particle is recorded. 

This method developed by C. T. R. Wilson in Cambridge, 
England, about 1912 has been enormously useful in studying the 
behavior of individual particles, alphas, protons, electrons, 
positrons, mesotrons, photons, and the fast atoms caused by 
collisions with alphas, protons, or neutrons. Unlike the scintilla- 

232 Appendix 1 

tion method, its companion tool for many years, it has not been 
superseded and is still used extensively, particularly to study 
details of collisions between nuclear particles and atoms. 


The tracks of individual particles passing through matter can 
also be observed in photographic emulsions, but the lengths of 
path are so small that they must be observed under a microscope, 
where they appear as a series of developed grains marking the 
passage of the particle. This method of observation requires 
practically no equipment but is tedious and of limited usefulness. 
It is possible, however, to use the general blackening of a 
photographic film as a measure of total exposure to radiation, 
a procedure that has been used to supplement or to replace 
electroscopes for safety control in many parts of the project. 


None of the methods we have described is directly applicable 
to neutrons, but all of them are indirectly applicable since neu- 
trons produce ions indirectly. This happens in two ways by 
elastic collision and by nuclear reaction. As we have already 
described, a fast neutron in passing through matter occasionally 
approaches an atomic nucleus so closely as to impart to it a large 
amount of momentum and energy according to the laws of 
elastic collision. The nucleus thereby becomes a high-speed 
charged particle which will produce ionization in an ionization 
chamber, counter, or cloud chamber. But if the neutron has low 
speed, e.g., thermal, the struck nucleus will not get enough 
energy to cause ionization. If, on the other hand, the neutron is 
absorbed and the resultant nucleus breaks up with the release of 
energy, ionization will be produced. Thus, for the detection of 
high-speed neutrons one has a choice between elastic collisions 
and nuclear reaction, but for thermal speeds only nuclear reaction 
will serve. 

The reaction most commonly used is the <>B 10 (n, a)sLi 7 reaction 
which releases about 2.5 Mev energy shared between the resultant 

Observing Fast Particles 233 

alpha particle and aLi 7 nucleus. This is ample to produce ioniza- 
tion. This reaction is used by filling an ionization chamber or 
proportional counter with boron trifluoride gas so that the reac- 
tion occurs in the region where ionization is wanted; as an alterna- 
tive the interior of the chamber or counter is lined with boron. 
The ionization chamber then serves as an instrument to measure 
overall neutron flux while the proportional counter records 
numbers of individual neutrons. 

One of the most valuable methods of measuring neutron densi- 
ties by nuclear reactions depends on the production of artificial 
radioactive nuclei. A foil known to be made radioactive by neu- 
tron bombardment is inserted at a point where the neutron 
intensity is wanted. After a given time it is removed and its 
activity measured by an electroscope or counter. The degree of 
activity that has been built up is then a measure of the number 
of neutrons that have been absorbed. This method has the 
obvious disadvantage that it does not give an instantaneous 
response as do the ionization chamber and counter. 

One of the most interesting methods developed on the project 
is to use the fission of uranium as the nuclear reaction for neutron 
detection. Furthermore, by separating the isotopes, fast and slow 
neutrons can be differentiated. 

Since the probability of a neutron reaction occurring is dif- 
ferent for every reaction and for every neutron speed, difficulties 
of translating counts or current measurements into numbers and 
speeds of neutrons present are even greater than for other nuclear 
particles. No one need be surprised if two able investigators give 
different numbers for supposedly the same nuclear consant. It 
is only by an intricate series of interlocking experiments carefully 
compared and interpreted that the fundamental facts can be 
untangled from experimental and instrumental variables. 



Since the proton and the neutron are the fundamental particles 
out of which all nuclei are built, it would seem natural to use the 
mass of one or the other of them as a unit of mass. The choice 
would probably be the proton, which is the nucleus of a hydrogen 
atom. There are good reasons, historical and otherwise, why 
neither the proton nor the neutron was chosen. Instead, the mass 
unit used in atomic and nuclear physics is one sixteenth of the 
mass of the predominant oxygen isotope, O 16 , and is equal to 
1.6603 X 10~ 24 gram. Expressed in terms of this unit, the mass 
of the proton is 1.00758 and the mass of the neutron is 1.00893. 
(Chemists usually use a very slightly different unit of mass.) 


The unit of electric charge used in nuclear science is the posi- 
tive charge of the proton. It is equal in magnitude but opposite 
in sign to the charge on the electron and is therefore often called 
the electronic charge. One electronic charge is 1.60 X 10~ 19 
coulomb. It may be recalled that a current of one ampere flowing 
for one second conveys a charge of one coulomb; i.e., one elec- 
tronic charge equals 1.60 X 10~ 19 ampere second. 


The energy unit used in nuclear physics is the electron volt, 
which is defined as equal to the kinetic energy which a particle 
carrying one electronic charge acquires in falling freely through 
a potential drop of one volt. It is often convenient to use the 
million-times greater unit: million electron volt (Mev). 


Units of Mass, Charge, Energy 


The relationships among the electron volt and other common 
units of energy are in the following table: 





g. cal. 

kw. hrs. 


1.07 X ID" 3 
1.60 X 10~ 6 
3.83 X 10~" 
4.45 X 10- 20 

9.31 X 10 2 
1.49 X 10~ 3 
3.56 X 10~" 

4.15 X 10~ 17 

6.71 X 10 2 

6.24 X 10 5 
2.39 X 10~ 8 
2.78 X 10-" 

2.81 X 10 10 
2.62 X 10 13 
4.18 X 10 7 

1.16 X 10~ 6 

2.41 X 10 16 

2.25 X 10 19 
3.60 X 10 13 
8.60 X 10 5 


mass units 
g. cal. 
kw. hrs. 

g. cal. 
kw. hrs. 

mass units 
g. cal. 
kw. hrs. 

mass units 
kw. hrs. 

mass units 
g. cal. 


As was pointed out in Chapter VI, the control of a chain- 
reacting pile is greatly facilitated by the fact that some of the 
neutrons resulting from uranium fission are not emitted until 
more than a second after fission occurs. It was therefore important 
to study this effect experimentally. Such experiments were 
described by Snell, Nedzel and Ibser in a report dated May 15, 
1942 from which we quote as follows: 

"The present experiment consists of two interrelated parts 
one concerned with the decay curve, and one concerned with the 
intensity of the delayed neutrons measured in terms of that of the 
'instantaneous' fission neutrons. 


"The neutron source was the beryllium target of the University 
of Chicago cyclotron struck by a beam of up to 20 pA of 8 Mev 
deuterons. Near the target was placed a hollow shell made of 
tinned iron and containing 106 Ibs. of UaOg. This was surrounded 
by about 2" of paraffin. The interior of the shell was filled with 
paraffin, except for an axial hole which accommodated a BF 3 - 
filled proportional counter. The counter was connected through 
an amplifier to a scaling circuit ('scale of 64') equipped with inter- 
polating lights and a Cenco impulse counter. A tenth-second 
timer, driven by a synchronous motor, and hundredth-second stop 
watch were mounted on the panel of the sealer, close to the 
interpolating lights and impulse counter. This group of dials and 
lights was photographed at an appropriately varying rate by a 
Sept camera which was actuated by hand. The result was a 
record on movie film of times and counts, from which the decay 
curves were plotted. 


Delayed Neutrons from Uranium Fission 237 

"The actual procedure was as follows: During bombardment 
the stop watch was started and the timer was running con- 
tinuously; the counter and amplifier were on, but the pulses 
leaving the amplifier were grounded. The sealer was set at zero. 
After a warning signal the cyclotron was shut off by one operator, 
while another operator switched the output of the amplifier from 
ground into the sealer, and started taking photographs. It was 
easy to take the first photograph within half a second of turning 
off the cyclotron. Sixty to a hundred photographs were taken 
during a typical run. The necessity of using both a stop watch 
and a timer arose from the fact that the hundredth-second pre- 
cision of the stop watch was needed for the small time intervals 
between photographs during the initial part of the run, but the 
watch ran down and stopped before the counting was complete. 
The timer then gave sufficient precision for the later time 

"Some forty runs were taken under varying experimental con- 
ditions. Short activations of one or two seconds were given for 
best resolution of the short periods. Long, intense bombardments 
lasting 15-20 minutes, as close as possible to the target, were 
made to make the long period activities show up with a maximum 
intensity. Some 5-minute bombardments were made, keeping the 
cyclotron beam as steady as possible, to study the relative satura- 
tion intensities of the various activities; in these activations the 
cyclotron beam was reduced to 1 or 2 /xA to prevent the initial 
counting rate from becoming too high for a counter (300 per sec. 
\\ as taken as a reasonable upper limit for reliable counting). Two 
BF 3 counters were available, one having a thermal neutron cross 
section of 2.66 sq. cm., and the other 0.43 sq. cm. After a strong 
activation, we could follow the decay of the delayed neutrons 
for some 13 minutes. Background counts (presumably chiefly 
due to spontaneous fission neutrons) were taken and were sub- 
tracted from the readings. They amounted to about 0.4 counts 
per sec. for the large counter. 

"A study of all the decay curves gives the following as a general 
picture of the neutron-emitting activities present: 

238 Appendix 3 



57 3 sec. 0.135 

24 2 sec. 1.0 

7 sec. 1.2 

2.5 sec. 1.2 

"Any activity of period longer than 57 sec. failed to appear 
even after the most intense bombardment we could give, lasting 
20 minutes. The relative initial intensities given are the average 
values obtained from three curves. 

"These results give the following equation for the decay curve, 
of the delayed neutrons after activation to saturation: 

Activity = constant (1.2e-- 28 < + 1.2e-- 099 ' + 1.0e-- 029 < 

-f 0.135e-- 012 
where t is in seconds." 

The second part of the experiment measured the total number 
of neutrons emitted in the time interval 0.01 sec. to 2.0 min. after 
the cyclotron was turned off. Assuming that all the delayed 
neutrons observed were in the four groups measured in the first 
part of the experiment, this second result indicated that 1.0 0.2 
per cent of the neutrons emitted in uranium fission are delayed 
by at least 0.01 sec. and that about 0.07 per cent are delayed by 
as much as a minute. By designing the effective value of k, the 
multiplication factor, for a typical operating pile to be only 1.01 
with all the controls removed and the total variation in k from 
one control rod to be 0.002, the number of delayed neutrons is 
sufficient to allow easy control. 


In Chapter VI the construction and operation of the first self- 
sustaining chain-reacting pile were described briefly. Though 
details must still be withheld for security reasons, the following 
paragraphs give a somewhat fuller description based on a report 
by Fermi. This pile was erected by Fermi and his collaborators in 
the fall of 1942. 


The original plan called for an approximately spherical pile 
with the best materials near the center. Actually control measure- 
ments showed that the critical size had been reached before the 
sphere was complete, and the construction was modified accord- 
ingly. The final structure may be roughly described as an oblate 
spheroid flattened at the top, i.e., like a door knob. It was desired 
to have the uranium or uranium oxide lumps spaced in a cubic 
lattice imbedded in graphite. Consequently, the graphite was 
cut in bricks and built up in layers, alternate ones of which con- 
tained lumps of uranium at the corners of squares. The critical 
size was reached when the pile had been built to a height only 
three quarters of that needed according to the most cautious 
estimates. Consequently only one more layer was added. The 
graphite used was chiefly from the National Carbon Company 
and the Speer Graphite Company. The pile contained 12,400 Ibs. 
of metal, part of which was supplied by Westinghouse, part by 
Metal Hydrides, and part by Ames. Since there were many more 
lattice points than lumps of metal, the remaining ones were filled 
with pressed oxide lumps. 

For purposes of control and experiment there were ten slots 
passing completely through the pile. Three of those near the 


240 Appendix 4 

center were used for control and safety rods. Further to facilitate 
experiment, particularly the removal of samples, one row of 
graphite bricks carrying uranium and passing near the center of 
the pile was arranged so that it could be pushed completely out 
of the pile. 

This whole graphite sphere was supported by a timber frame- 
work resting on the floor of a squash cpurt under the West Stands 
of Stagg Field. 


The metal lattice at the center of the pile and the two other 
major lattices making up the bulk of the rest of the pile had each 
been studied separately in exponential experiments #18, #27, and 
#29. These had given a multiplication factor of 1.07 for the metal 
lattice and 1.04 and 1.03 for the oxide lattices, the difference in 
the last two resulting from difference in the grade of graphite used. 
It is to be remembered that these figures are multiplication factors 
for lattices of infinite size. Therefore a prediction of the actual 
effective multiplication factor k e ff for the pile as constructed 
depended on the validity of the deduction of k from the ex- 
ponential experiments, on a proper averaging for the different 
lattices, and on a proper deduction of k eff from the average k for 
infinite size. Although the original design of the pile had been 
deliberately generous, its success when only partly completed 
indicated that the values of the multiplication factors as calculated 
from exponential experiments had been too low. The observed 
effective multiplication factor of the part of the planned structure 
actually built was about 1 .0006 when all neutron absorbers were 


A series of measurements was made while the pile was being 
assembled in order to be sure that the critical dimensions were 
not reached inadvertently. These measurements served also to 
check the neutron multiplication properties of the structure 

First Self-Sustaining Pile 


during assembly, making possible a prediction of where the 
critical point would be reached. 

In general, any detector of neutrons or gamma radiation can 
be used for measuring the intensity of the reaction. Neutron 
detectors are somewhat preferable since they give response more 





quickly and are not affected by fission-product radiations after 
shut down. Actually both neutron detectors (boron trifluoride 
counters) and gamma-ray ionization chambers were distributed 
in and around the pile. Certain of the ionization chambers were 
used to operate recording instruments and automatic safety 

In the pile itself measurements were made with two types of 
detector. A boron trifluoride counter was inserted in a slot about 

242 Appendix 4 

43" from the ground and its readings taken at frequent intervals. 
In addition, an indium foil was irradiated every night in a posi- 
tion as close as possible to the effective center of the pile, and its 
induced activity was measured the following morning and com- 
pared with the readings of the boron trifluoride counter. 

The results of such measurements can be expressed in two ways. 
Since the number of secondary neutrons produced by fission will 
increase steadily as the pile is constructed, the activity A induced 
in a standard indium foil at the center will increase steadily as 
the number of layers of the pile is increased. Once the effective 
multiplication factor is above one, A would theoretically increase 
to infinity. Such an approach to infinity is hard to observe, so a 
second way of expressing the results was used. Suppose the lattice 
spacing and purity of materials of a graphite-uranium structure 
are such that the multiplication factor would be exactly one if 
the structure were a sphere of infinite radius. Then, for an actual 
sphere of similar construction but finite radius, the activation of a 
detector placed at the center would be proportional to the square 
of the radius. It was possible to determine a corresponding 
effective radius Rff for the real pile in each of its various stages. 
It followed, therefore, that, if the factor k^ were precisely one on 
the average for the lattice in the pile, the activity A of the de- 
tector at the center should increase with increasing R e ff in such 
a way that R 2 ef f/A remained constant, but, if k*, for the lattice 
were greater than one, then as the pile size approached the 
critical value, that is, as k^ff approached one, A should approach 
infinity and therefore R 2 e ff/A approach zero. Therefore by extra- 
polating a curve of R 2 e ff/A vs. size of the pile i.e., number of layers, 
to where it cut the axis, it was possible to predict at what layer 
k e ff would become one. Such a curve, shown in Fig. 1, indicated 
at what layer the critical size would be reached. The less useful 
but more direct and dramatic way of recording the results is 
shown in Fig. 2, which shows the growth of the neutron activity 
of the pile as layers were added. 

During the construction, appreciably before reaching this 
critical layer, some cadmium strips were inserted in suitable slots. 

First Self -Sustaining Pile 










244 Appendix 4 

They were removed once every day with the proper precautions 
in order to check the approach to the critical conditions. The 
construction was carried in this way to the critical layer. 


The reaction was controlled by inserting in the pile some strips 
of neutron-absorbing material cadmium or boron steel. When 
the pile was not in operation, several such cadmium strips were 
inserted in a number of slots, bringing the effective multiplication 
factor considerably below one. In fact, any one of the cadmium 
strips alone was sufficient to bring the pile below the critical 
condition. Besides cadmium strips that could be used for manual 
operation of the pile, two safety rods and one automatic control 
rod were provided. The automatic control rod was operated by 
two electric motors responding to an ionization chamber and 
amplifying system so that, if the intensity of the reaction increased 
above the desired level, the rod was pushed in, and vice versa. 


To operate the pile all but one of the cadmium strips were 
taken out. The remaining one was then slowly pulled out. As 
the critical conditions were approached, the intensity of the 
neutrons emitted by the pile began to increase rapidly. It should 
be noticed, however, that, when this last strip of cadmium was so 
far inside the pile that the effective multiplication factor was just 
below one, it took a rather long time for the intensity to reach 
the saturation value. Similarly, if the cadmium strip was just 
far enough out to make k e ff greater than one, the intensity rose 
at a rather slow rate. For example, if one rod is only 1 cm. out 
from the critical position, the "relaxation time," i.e., the time 
for the intensity to double, is about four hours. These long 
"relaxation times" were the result of the small percentage of 
delayed neutrons which have been discussed in Appendix 3, and 
make it relatively easy to keep the pile operating at a constant 
level of intensity. 

First Self-sustaining Pile 245 

The pile was first operated on December 2, 1 942 to a maximum 
energy production of about J^ watt. On December 12th the 
intensity was run up to about 200 watts, but it was not felt safe 
to go higher because of the danger of the radiation to personnel 
in and around the building. During this high intensity run, 
measurements were made of radiation intensity beside the pile, 
in the building, and on the sidewalk outside. 


Presented below is a list of titles of representative reports pre- 
pared in the Metallurgical Laboratory of the University of Chi- 
cago in 1942. 

A Table for Calculating the Percentage Loss Due to the 

Presence of Impurities in Alloy 

Concerning the Radium-Beryllium Neutron Sources 
Preliminary Estimates of the Radiations from Fission 


Background of Natural Neutrons in Multiplying Pile 
Absorption Cross Sections for Rn plus Be Fast Neutrons 
On Mechanical Stresses Produced by Temperature Gra- 
dients in Rods and Spheres 

Effect of Geometry on Resonance Absorption of Neutrons by 

Protection against Radiations 

Planning Experiments on Liquid Cooling 

Report on the Possibility of Purifying Uranium by Carbonyl 

Formation and Decomposition 
On the Radioactivity of Cooling Helium 
Estimation of Stability of Ether under Various Conditions of 

Uranium Poisoning 

Transuranic and Fission Product Activities 
Chemical Effects of Radiation on Air Surrounding the Pile 
An Estimate of the Chemical Effects of Radiation on the 

Cooling Water in the Pile 

The Extraction Method of Purification of Uranyl Nitrate 
The Diffusion of Fission Products from Cast Metal at 600C 

and 1000C 



"Mankind's successful transition to a new age, the Atomic Age, 
was ushered in July 16, 1945, before the eyes of a tense group of 
renowned scientists and military men gathered in the desertlands 
of New Mexico to witness the first end results of their $2,000,000,- 
000 effort. Here in a remote section of the Alamogordo Air Base 
120 miles southeast of Albuquerque the first man-made atomic 
explosion, the outstanding achievement of nuclear science, was 
achieved at 5: 30 a.m. of that day. Darkening heavens, pouring 
forth ram and lightning immediately up to the zero hour, height- 
ened the drama. 

"Mounted on a steel tower, a revolutionary weapon destined 
to change war as we know it, or which may even be the instru- 
mentality to end all wars, was set off with an impact which 
signalized man's entrance into a new physical world. Success was 
greater than the most ambitious estimates. A small amount of 
matter, the product of a chain of huge specially constructed 
industrial plants, was made to release the energy of the universe 
locked up within the atom from the beginning of time. A fabulous 
achievement had been reached. Speculative theory, barely estab- 
lished in pre-war laboratories, had been projected into practicality. 

"This phase of the Atomic Bomb Project, which is headed by 
Major General Leslie R. Groves, was under the direction of Dr. 
J. R. Oppenheimer, theoretical physicist of the University of 
California. He is to be credited with achieving the implementa- 
tion of atomic energy for military purposes. 

"Tension before the actual detonation was at a tremendous 
pitch. Failure was an ever-present possibility. Too great a success, 
envisioned by some of those present, might have meant an uncon- 
trollable, unusable weapon. 

"Final assembly of the atomic bomb began on the night of 


248 Appendix 6 

July 12 in an old ranch house. As various component assemblies 
arrived from distant points, tension among the scientists rose to 
an increasing pitch. Coolest of all was the man charged with the 
actual assembly of the vital core, Dr. R. F. Bacher, in normal 
times a professor at Cornell University. 

"The entire cost of the project, representing the erection of 
whole cities and radically new plants spread over many miles of 
countryside, plus unprecedented experimentation, was repre- 
sented in the pilot bomb and its parts. Here was the focal point 
of the venture. No other country in the world had been capable 
of such an outlay in brains and technical effort. 

"The full significance of these closing moments before the final 
factual test was not lost on these men of science. They fully knew 
their position as pioneers into another age. They also knew that 
one false move would blast them and their entire effort into 
eternity. Before the assembly started a receipt for the vital matter 
was signed by Brigadier General Thomas F. Farrell, General 
Groves' deputy. This signalized the formal transfer of the irre- 
placeable material from the scientists to the Army. 

"During final preliminary assembly, a bad few minutes devel- 
oped when the assembly of an important section of the bomb was 
delayed. The entire unit was machine- tooled to the finest meas- 
urement. The insertion was partially completed when it appar- 
ently wedged tightly and would go no farther. Dr. Bacher, 
however, was undismayed and reassured the group that time 
would solve the problem. In three minutes' time, Dr. Bacher's 
statement was verified and basic assembly was completed without 
further incident. 

"Specialty teams, comprised of the top men on specific phases 
of science, all of which were bound up in the whole, took over 
their specialized parts of the assembly. In each group was central- 
ized months and even years of channelized endeavor. 

"On Saturday, July 14, the unit which was to determine the 
success or failure of the entire project was elevated to the top of 
the steel tower. All that day and the next, the job of preparation 
went on. In addition to the apparatus necessary to cause the 

New Mexico Test, July 16, 1945 249 

detonation, complete instrumentation to determine the pulse 
beat and all reactions of the bomb was rigged on the tower. 

"The ominous weather which had dogged the assembly of the 
bomb had a very sobering affect on the assembled experts whose 
work was accomplished amid lightning flashes and peals of 
thunder. The weather, unusual and upsetting, blocked out aerial 
observation of the test. It even held up the actual explosion 
scheduled at 4:00 a.m. for an hour and a half. For many months 
the approximate date and time had been set and had been one 
of the high-level secrets of the best kept secret of the entire war. 

"Nearest observation point was set up 10,000 yards south of 
the tower where in a timber and earth shelter the controls for the 
test were located. At a point 17,000 yards from the tower at a 
point which would give the best observation the key figures in 
the atomic bomb project took their posts. These included General 
Groves, Dr. Vannevar Bush, head of the Office of Scientific 
Research and Development and Dr. James B. Conant, president 
of Harvard University. 

"Actual detonation was in charge of Dr. K. T. Bainbridge of 
Massachusetts Institute of Technology. He and Lieutenant Bush, 
in charge of the Military Police Detachment, were the last men 
to inspect the tower with its cosmic bomb. 

"At three o'clock in the morning the party moved forward to 
the control station. General Groves and Dr. Oppenheimer con- 
sulted with the weathermen. The decision was made to go ahead 
with the test despite the lack of assurance of favorable weather. 
The time was set for 5:30 a.m. 

"General Groves rejoined Dr. Conant and Dr. Bush, and just 
before the test time they joined the many scientists gathered at 
the Base Camp. Here all present were ordered to lie on the 
ground, face downward, heads away from the blast direction. 

"Tension reached a tremendous pitch in the control room as 
the deadline approached. The several observation points in the 
area were tied in to the control room by radio and with twenty 
minutes to go, Dr. S. K. Allison of Chicago University took over 
the radio net and made periodic time announcements. 

250 Appendix 6 

"The time signals, 'minus 20 minutes, minus fifteen minutes,' 
and on and on increased the tension to the breaking point as the 
group in the control room which included Dr. Oppenheimer and 
General Farrell held their breaths, all praying with the intensity 
of the moment which will live forever with each man who was 
there. At 'minus 45 seconds/ robot mechanism took over and 
from that point on the whole great complicated mass of intricate 
mechanism was in operation without human control. Stationed 
at a reserve switch, however, was a soldier scientist ready to 
attempt to stop the explosion should the order be issued. The 
order never came. 

"At the appointed time there was a blinding flash lighting up 
the whole area brighter than the brightest daylight. A mountain 
range three miles from the observation point stood out in bold 
relief. Then came a tremendous sustained roar and a heavy 
pressure wave which knocked down two men outside the control 
center. Immediately thereafter, a huge multi-colored surging 
cloud boiled to an altitude of over 40,000 feet. Clouds in its path 
disappeared. Soon the shifting substratosphere winds dispersed 
the now grey mass. 

"The test was over, the project a success. 

"The steel tower had been entirely vaporized. Where the tower 
had stood, there was a huge sloping crater. Dazed but relieved at 
the success of their tests, the scientists promptly marshalled their 
forces to estimate the strength of America's new weapon. To 
examine the nature of the crater, specially equipped tanks were 
wheeled into the area, one of which carried Dr. Enrico Fermi, 
noted nuclear scientist. Answer to their findings rests in the 
destruction effected in Japan today in the first military use of the 
atomic bomb. 

"Had it not been for the desolated area where the test was held 
and for the cooperation of the press in the area, it is certain that 
the test itself would have attracted far-reaching attention. As it 
was, many people in that area are still discussing the effect of the 
smash. A significant aspect, recorded by the press, was the experi- 
ence of a blind girl near Albuquerque many miles from the 

New Mexico Test, July 76, 7945 251 

scene, who, when the flash of the test lighted the sky before the 
explosion could be heard, exclaimed, 'What was that?' 

"Interviews of General Groves and General Farrell give the 
following on-the-scene versions of the test. General Groves said: 
'My impressions of the night's high points follow: After about an 
hour's sleep I got up at 0100 and from that time on until about 
five I was with Dr. Oppenheimer constantly. Naturally he was 
tense, although his mind was working at its usual extraordinary 
efficiency. I attempted to shield him from the evident concern 
shown by many of his assistants who were disturbed by the un- 
certain weather conditions. By 0330 we decided that we could 
probably fire at 0530. By 0400 the rain had stopped but the sky 
was heavily overcast. Our decision became firmer as time 
went on. 

" ' During most of these hours the two of us journeyed from the 
control house out into the darkness to look at the stars and to 
assure each other that the one or two visible stars were becoming 
brighter. At 0510 I left Dr. Oppenheimer and returned to the 
main observation point which was 17,000 yards from the point 
of explosion. In accordance with our orders I found all personnel 
not otherwise occupied massed on a bit of high ground. 

" 'Two minutes before the scheduled firing time, all persons lay 
face down with their feet pointing towards the explosion. As the 
remaining time was called from the loud speaker from the 10,000- 
yard control station there was complete awesome silence. Dr. 
Conant said he had never imagined seconds could be so long. 
Most of the individuals in accordance with orders shielded their 
eyes in one way or another. 

" 'First came the burst of light of a brilliance beyond any com- 
parison. We all rolled over and looked through dark glasses at 
the ball of fire. About forty seconds later came the shock wave 
followed by the sound, neither of which seemed startling after 
our complete astonishment at the extraordinary lighting intensity. 

" 'A massive cloud was formed which surged and billowed 
upward with tremendous power, reaching the substratosphere in 
about five minutes. 

252 Appendix 6 

" 'Two supplementary explosions of minor effect other than the 
lighting occurred in the cloud shortly after the main explosion. 

" 'The cloud traveled to a great height first in the form of a 
ball, then mushroomed, then changed into a long trailing chim- 
ney-shaped column and finally was sent in several directions by 
the variable winds at the different elevations. 

" 'Dr. Conant reached over and we shook hands in mutual con- 
gratulations. Dr. Bush, who was on the other side of me, did 
likewise. The feeling of the entire assembly, even the uninitiated, 
was of profound awe. Drs. Conant and Bush and myself were 
struck by an even stronger feeling that the faith of those who had 
been responsible for the initiation and the carrying on of this 
Herculean project had been justified." 

General FarrelPs impressions are: "The scene inside the shelter 
was dramatic beyond words. In and around the shelter were some 
twenty odd people concerned with last-minute arrangements. 
Included were Dr. Oppenheimer, the Director who had borne 
the great scientific burden of developing the weapon from the 
raw materials made in Tennessee and Washington, and a dozen 
of his key assistants, Dr. Kistiakowsky, Dr. Bainbridge, who super- 
vised all the detailed arrangements for the test; the weather 
expert, and several others. Besides those, there were a handful 
of soldiers, two or three Army officers and one Naval Officer. 
The shelter was filled with a great variety of instruments and 

" 'For some hectic two hours preceding the blast, General 
Groves stayed with the Director. Twenty minutes before the 
zero hour, General Groves left for his station at the base camp, 
first because it provided a better observation point and second, 
because of our rule that he and I must not be together in situa- 
tions where there is an element of danger which existed at both 

" 'Just after General Groves left, announcements began to be 
broadcast of the interval remaining before the blast to the other 
groups participating in and observing the test. As the time interval 
grew smaller and changed from minutes to seconds, the tension 

New Mexico Test, July 16, 1945 253 

increased by leaps and bounds. Everyone in that room knew the 
awful potentialities of the thing that they thought was about to 
happen. The scientists felt that their figuring must be right and 
that the bomb had to go off but there was in everyone's mind a 
strong measure of doubt. 

" 'We were reaching into the unknown and we did not know 
what might come of it. It can safely be said that most of those 
present were praying and praying harder than they had ever 
prayed before. If the shot were successful, it was a justification 
of the several years of intensive effort of tens of thousands of 
people statesmen, scientists, engineers, manufacturers, soldiers, 
and many others in every walk of life. 

"'In that brief instant in the remote New Mexico desert, the 
tremendous effort of the brains and brawn of all these people 
came suddenly and startlingly to the fullest fruition. Dr. Oppen- 
heimer, on whom had rested a very heavy burden, grew tenser 
as the last seconds ticked off. He scarcely breathed. He held on 
to a post to steady himself. For the last few seconds, he stared 
directly ahead and then when the announcer shouted "Now!" 
and there came this tremendous burst of light followed shortly 
thereafter by the deep growling roar of the explosion, his face 
relaxed into an expression of tremendous relief. Several of the 
observers standing back of the shelter to watch the lighting effects 
were knocked flat by the blast. 

" 'The tension in the room let up and all started congratulating 
each other. Everyone sensed 'This is it!'. No matter what might 
happen now all knew that the impossible scientific job had been 
done. Atomic fission would no longer be hidden in the cloisters 
of the theoretical physicists' dreams. It was almost full grown at 
birth. It was a great new force to be used for good or for evil. 
There was a feeling in that shelter that those concerned with its 
nativity should dedicate their lives to the mission that it would 
always be used for good and never for evil. 

" 'Dr. Kistiakowsky threw his arms around Dr. Oppenheimer 
and embraced him with shouts of glee. Others were equally 
enthusiastic. All the pent-up emotions were released in those few 

254 Appendix 6 

minutes and all seemed to sense immediately that the explosion 
had far exceeded the most optimistic expectations and wildest 
hopes of the scientists. All seemed to feel that they had been 
present at the birth of a new age The Age of Atomic Energy 
and felt their profound responsibility to help in guiding into 
right channels the tremendous forces which had been unlocked 
for the first time in history. 

"'As to the present war, there was a feeling that no matter 
what else might happen, we now had the means to insure its 
speedy conclusion and save thousands of American lives. As to 
the future, there had been brought into being something big 
and something new that would prove to be immeasurably more 
important than the discovery of electricity or any of the other 
great discoveries which have so affected our existence. 

" 'The effects could well be called unprecedented, magnificent, 
beautiful, stupendous and terrifying. No man-made phenomenon 
of such tremendous power had ever occurred before. The lighting 
effects beggared description. The whole country was lighted by a 
searing light with the intensity many times that of the midday 
sun. It was golden, purple, violet, gray and blue. It lighted every 
peak, crevasse and ridge of the nearby mountain range with a 
clarity and beauty that cannot be described but must be seen to 
be imagined. It was that beauty the great poets dream about but 
describe most poorly and inadequately. Thirty seconds after, the 
explosion came first, the air blast pressing hard against the people 
and things, to be followed almost immediately by the strong, 
sustained, awesome roar which warned of doomsday and made 
us feel that we puny things were blasphemous to dare tamper with 
the forces heretofore reserved to the Almighty. Words are inade- 
quate tools for the job of acquainting those not present with the 
physical, mental and psychological effects. It had to be witnessed 
to be realized.' " 



References are to chapter and paragraph except references to appendices which 
are shown as A-7 etc. 

Abelson, P. H., 4:36, 6:34, 11:38, 


Adamson, Col. K. F., 3:4, 3:5, 3:6 
Adler, E., 10:26 
Aebersold, P. C., 11:6 
Akers, W. A., 5:14 
Allison, S. K., 3:14, 4:18, 4:19, 

4:20, 4:43, 5:2, 5:4, 5:18, 6:5, 

6:22, 7:18, 12:23, A-6 
Anderson, C. D., 1 :21 
Anderson, H. L., 4:2, 4:13, 6:31, 


Arnold, H. R., 9:40 
Arnold, J. H., 10:24 
Aston, F. W., 1:35, 9:15 

Bacher, R. F., 12:23, A-6 

Backus, J. G., 11:6 

Baker, A. L., 10:24 

Barr, F. T., 9:41 

Baxter, J. W., 11:34 

Beams, J. W., 3:3, 3:9, 3:14, 5:2, 

5:4, 5:12, 5:18, 9:18 
Beans, H. T., 4:44 
Becker, H., 1:19 
Becquerel, H., 1:6, 1:7 
Benedict, M., 9:32, 10:3 
Bethe, H., 1:51, 12:23, 12:24 
Bohr, N., 1:53,1:57,3:2,12:23 
Boorse, H. A., 10:28 
Booth, E. T., 10:1, 10:2, 10:7, 

Bothe, W., 1:19 

Bowen, Adm. H. G., 3:6, 3:7 

Boyd, G. E., 6:33 

Breit, G., 3:3, 3:8, 3:9, 3:14, 4:22, 
5:2, 5:4, 5:18, 6:37, 6:38, 12:2 

Brewer, A. K., 9:31 

Brewster, O. C., 10:28 

Brickwedde, F. G., 1:21 

Briggs,L.J,3:3,3:4 5 3:5,3:6,3:7, 
3:8, 3:9, 3:14, 3:15, 3:16, 4:3, 
4:36, 5:2, 5:4, 5:6, 5:8, 5:14, 
5:17, 10:2 

Brobeck, F. A., 11:6 

Buckley, O. E., 3:16 

Bush, V., 3:9, 3:15, 3:16, 3:17, 
3:18, 3:19, 3:21, 3:22, 5:2, 5:4, 
5:8, 5:9, 5:13, 5:14, 5:17, 5:20, 
5:21, 5:22, 5:25, 5:26, 5:27, 
5:28, 5:33, A-6 

Cady, G. H., 10:29 
Cantril, S. T., 7:48 
Carpenter, W. S., Jr., 7:5 
Chadwick, J., 1:19, 3:17, 4:47, 


Chapman, S., 9:21 
Chilton, T. H., 3:14 
Chubb, L. W., 3:16, 5:5 
Clark, W. M., 3:3 
Clusius, H., 9:22 
Cockcroft, J. D., 1:38, 1:40, 3:17 
Cohen, K., 4:34, 9:32, 10:3, 10:7 
Cole, Dr., 8:70 




Compton, A. H., 3:15, 3:16, 3:19, 
4:18, 5:4, 5:7, 5:8, 5:12, 5:14, 
5:17, 5:21, 5:31, 6:2, 6:8, 6:22, 
7:1, 7:48 

Conant, J. B., 3:16, 3:18, 3:19, 
5:4, 5:6, 5:8, 5:9, 5:10, 5:14, 
5:17, 5:21, 5:25, 5:27, 5:31, 
5:33, 12:5, A-6 

Condon, E. U., 3:14, 5:2, 5:4, 

Cooksey, D., 11:6 

Coolidge, W. D., 3:15, 3:16 

Cooper, C. M., 6:5, 7:48 

Coryell, C. D., 6:33 

Creutz, E. C., 4:12, 6:36 

Crist, R. H., 9:40 

Curie, I., 1:19,1:27 

Curie, M., 1:6 

Curie, P., 1:6 

Curme, G. O., Jr., 5:5 

Currie, L. M., 10:23 

Daniels, F., 7:48 
Dempster, A. J., 7:48, 9:29 
Dickel, G., 9:22 
Doan, R. L., 6:5, 6:8, 7:48 
Dunning, J. R., 1:53, 10:1, 10:2, 
10:23, 10:41 

Eckart, C. H., 3:14, 4:22 
Einstein, A., 1:4, 1:9, 1:39, 1:53, 


Eisenhart, L. P., 3:3 
Emmett, P. H., 10:23 
Enskog, D., 9:21 

Farrell, Gen. T. F., 5:32, A-6 

Feld, B., 4:13 

Fermi, E., 1:28, 1:52, 2:1, 2:10, 
2:11, 3:1, 3:4, 3:6, 3:8, 3:14, 
4:2, 4:13, 4:17, 4:22, 5:12, 6:5, 
6:22, 6:31, 7:48, 8:27, 12:23, 
A-4, A-6 

Feynman, R. P., 9:32 
Finkelstein, T., 11:6 
Fletcher, H., 3:3 
Fowler, R. H., 3:17, 4:38 
Franck,J., 6:5, 7:48 
Fred, E. B., 3:3 
Frisch, O. R., 1:53, 1:54 

Gary, T. C., 6:6 

Gherardi, B., 3:15 

Greenewalt, C. H., 6:6, 7:50 

Grosse, A. von, 4:39, 9:40, 10:1, 

Groves, Gen. L. R., 5:24, 5:25, 
5:27, 5:28, 5:31, 5:32, 5:33, 
7:4, 7:5, 7:12, 8:50, 10:34, 
11:18, 12:2, 12:5, 13:5, 13:6, 

Gunn, R., 3:9, 4:36, 5:2 

Hahn, O., 1:53, 1:54 

Halban, H., 5:14 

Hamilton, Dr., 8:70 

Helmholz, A. C., 11:6 

Henne, A. L., 10:29 

Heydenburg, N. P., 4:29 

Hertz, G., 9:15 

Hilberry, N., 6:8, 6:20, 6:33, 

6:45, 7:48 

Hoffman, J. I., 6:12 
Hogness, T., 6:5, 7:48 
Hoover, Comm. G. C., 3:4, 3:5, 


Jenkins, F. A., 11:6 
Jewett, F. B., 3:15 
Johns, I. B., 6:33 
Johnson, C. A., 10:26 
Johnson, W. C., 6:5, 7:48 
Joliot, F., 1:19, 1:27, 1:53, 3:2 

Kaplan, L, 9:32 
Keith, P. C., 5:5, 10:24 



Kennedy, J. W., 4:24, 6:33, 6:34, 

12:23, 12:48 
Kingdon, K. H., 11:6 
Kistiakowski, G. B., 3:16, 12:23, 


Langmuir, I., 11:6 

Lauritsen, C. C., I 

Lawrence, E. O., 3:15, 3:16, 3:17, 
5:7, 5:8, 5:17, 5:21, 5:31, 6:2, 
11:1, 11:2, 11:4, 11:5, 11:6, 
11:26, 11:29, 11:45 

Leverett, M. C., 6:5, 7:15 

Lewis, W. K., 3:16, 5:5, 6:6, 13:5 

Libby, W. F., 10:2 

McBee, E. T., 10:29 

Mack, E., 10:23 

MacKenzie, K. R., 11:6 

McMillan, E., 6:34 

Manley,J. H., 6:39 

Marshall, Gen. G. C., 5:9, 5:21, 

Marshall, Col. J. C., 5:23, 5:28, 


Matthias, Col. F. T., 5:32 
Meitner, L., 1:53, 1:54 
Miles, J. B., 7:50 
Miller, W. T., 10:29 
Mills, Adm. E. W., 13:5 
Mitchell, A. C. G., 4:11 
Mohler, F. L., 3:5 
Moore, T. W., 6:5, 7:15 
Moses, Gen. R. G., 5:26 
Mulliken, R. S., 3:16 
Murphree, E. V., 5:4, 5:5, 5:8, 

5:17, 6:6, 6:8, 9:41, 10:2 
Murphy, G. M., 1:21, 10:23 

Nichols, Col. K. D., 5:32, 10:34 
Nier, A. O., 4:29, 10:1, 10:29, 11 :4 

Nix, F. C., 10:26 
Norris, E. O., 10:26 

Oliphant, M. L. E., 3:19, 11:6, 


Oppenheimer, F., 11:6 
Oppenheimer, J. R., 5:31, 6:37, 

6:39, 11:6, 12:2, 12:5, 12:23, 

12:50, A-6 

Parkins, W. E., 11:6 
Parsons, Capt. W. S., 12:23 
Paxton, H. C., 10:7, 10:26 
Pegram, G. B., 1:53, 3:3, 3:4, 3:6, 

3:8, 3:9, 3:14, 3:20, 3:21, 3:22, 

4:2, 4:3, 4:5, 4:6, 4:46, 5:2, 5:4, 


Peierls, R., 10:4 
Perlman, L, 6:33 
Peters, B., 11:6 
Purnell, Adm. W. R., 5:25, 5:26, 

5:27, 5:33 

Rayleigh, Lord, 9:14 
Reynolds, W. B., 11:6 
Richardson, J. R., 11:6 
Roberts, R. B., 3:5 
Rodden, C.J., 4:44, 6:16 
Roosevelt, President F. D., 3:4, 

3:5, 3:6, 3:7, 3:9, 3:22, 3:23, 

5:25, 5:33, 7:5 
Rosen, R., 10:29 
Ruhoff, Col., 6:19 
Rutherford, E., 1:6, 1:17, 1:38, 


Sachs, A., 3:4, 3:5, 3:6, 3:7 
Seaborg, G. T., 4:24, 6:33, 6:34 
Segre, E., 4:24 
Serber, R., 6:39 
Simon, F., 5:14, 10:4 
Slack, F. G., 10:2, 10:26 



Slade, C. B., 10:7, 10:26 

Slater, J. C., 3:15 

Slepian,J., 11:2,11:6, 11:29 

Sloan, D. H., 11:6 

Smith, C. S., 12:23, 12:48 

Smith, L.S., 11:6, 11:29 

Smyth, H. D., 3:14, 4:12, 4:27, 

4:28, 4:31, 5:2, 5:4, 5:18, 7:48, 

8:56, 11:1, 11:24, 11:29, 13:5 
Snell, A-3 

Solberg, Capt. T. A., 13:5 
Somervell, Gen. B., 5:25 
Spedding, F. H., 6:5, 6:15, 6:17, 


Stearns, J. C., 7:48 
Squires, A. M., 9:32 
Stewart, I., 5:17 

Stimson, H. L., 5:9, 5:21, 5:25, 7:5 
Stone, Dr. R. S., 6:5, 7:48, 8:70 
Strassmann, F., 1:53, 1:54 
Steyer, Gen. W. D., 5:21, 5:22, 

5:25, 5:27, 5:33 
Swearingen, J. S., 10:28 
Szilard, L., 2:10, 2:11, 3:1, 3:4, 

3:5, 3:6, 3:8, 3:14, 4:2 

Taylor, H. S., 9:40, 10:23, 10:26 
Teller, E., 3:1, 3:5, 3:9, 6:39 
Thiele, E. W., 6:33 
Thomas, C. H., 7:48 
Thomson, G. P., 3:18 
Thomson, J. J., 1 :35 
Thornton, R. L., 11:6 
Tolman, R. C., 5:31, 13:5 
Truman, President H. S., 5:34 
Turner, L. A., 1:53, 1:58 
Tuve, M. A., 3:8, 3:9, 4:29 

Urey, H. C., 1:21, 3:3, 3:7, 3:8, 
3:9, 3:12, 3:14, 3:20, 3:21, 
3:22, 3:23, 4:32, 4:38, 4:46, 
5:2, 5:4, 5:7, 5:8, 5:12, 5:17, 
5:21, 5:31, 6:2, 6:33, 9:18, 
10:2, 10:23, 10:41 

VanVleck,J. H., 3:15 
Vernon, H. C., 7:48, 8:56 

Wahl, A. C., 4:24, 6:33, 6:34 
Wallace, H. A., 3:22, 5:9, 5:21 
Walton, E. T. S., 1:38, 1:40 
Warren, Col. S. L., 5:32 
Watson, Gen. E. M., see Roosevelt, 

President F. D. 
Watson, W. W., 7:48 
Watts, G. W., 10:28 
Weil, G., 4:13, 8:27 
Weisskopf, V. F., 3:1 
Weed, L. H., 3:3 
Wensel, H. T., 5:2, 5:4, 5:17 
Wever, E. G., 3:3 
Wheeler, J. A., 1:53, 1:57, 3:14, 

Whitaker, M. D., 7:48, 7:50, 8:34 
Wigner, E., 3:1, 3:3, 3:4, 3:5, 3:6, 

3:8, 3:9, 4:12, 4:22, 4:27, 4:28, 

6:5, 6:33, 7:15, 7:48, 8:56 
Wilhelm, H. A., 6:33 
Williams, R., 6:6, 7:50 
Wilson, C. T. R., A-l 
Wilson, R. R., 4:12, 11:24, 12:23 
Wilson, V. C., 6:31 

Zinn, W. H., 4:13, 6:31, 8:27, 8:61 


Actinium, 1:15, 1:16 

Advisory Committee on Uranium, 
3:4, 3:5, 3:8, 3:9. See also Urani- 
um Committee (NDRC), ND- 
RC Section S-l, OSRD Section 
S-l, OSRD Section S-l Execu- 
tive Committee 

Allis-Chalmers Manufacturing 
Company, 11:19, 11:45 

Alpha particle, 1:33, 2:32, 4:24; 
bombardment by 1:17, 1:19, 
1:40; product of nuclear disin- 
tegration; 1:9, 1:15, 1:16, 1:38, 
1:51, A-l 

Aluminum, 7:22 

Argonne Laboratory, 8:27, 8:28, 

Atomic number, 1:12, 1:15 

Atomic structure, 1:6, 1:10, 1:11 

Bakelite Corp., 10:26 

Barium, 1:54 

Bell Telephone Laboratories, Inc., 

Beryllium, 1:19, 1:40, 1:45, 2:10, 
2:13, 2:29, 2:36, 4:10, 4:14, 
4:18, 4:19, 4:43, 6:8, 6:22 

Beta particles, 1:9, 1:15, 1:16, 
2:32, 4:24, 6:34, 8:17, A-l 

Binding energies, nuclear, l:31ff., 
1:36, 1:54 

Bomb, atomic, 2:1, 2:3, 2:14, 
2:16, 2:23, 2:34, 2:35, 2:36, 3:5, 
4:25, 4:48, 4:49, 5:2, 5:10, 5:16, 
6:1, 6:32, 6:37, 11:28, 11:42, 
12:1, 13:1, A-6 

Boron, 1:19, 1:40, 2:10, 4:42, 
6:11, A-l, A-3 

Cadmium, 3:11, 4:6, 7:27, 8:8, A-4 
California Institute of Technology, 

California, University of, 1:53, 

3:12, 4:7, 4:24, 4:25, 6:33, 

6:34, 6:38, 11:2, 11:6, 11:12, 

11:19,11:23,11:44, 11:45, 12:7, 

Canadian Radium and Uranium 

Corp., 6:10 
Carbide and Carbon Chemicals 

Corp., 10:2, 10:25, 10:11 
Carbon, 1:51, 2:10, 2:13, 3:5, 3:6, 

3:8, 4:2 
Carnegie Institute of Washington, 

Centrifugal isotope separation, 

4:32, 4:37, 4:49, 5:6, 5:15, 

9:18,9:36,9:42, 11:16 
Chain reaction, 1:50, 1:51, 1:56, 

2:3ff., 2:23, 2:31, 2:32, 2:33, 

2:34, 2:36, 4:1, 4:2ff., 4:47, 

5:7, 6:6, 6:21ff., 8:3ff., 12:8 
Chemical analysis, 4:44, 7:34 
Chemical exchange method of 

isotope separation, 4:39, 9:24, 

Chemical problems, 7:39, 11:35, 

Chicago, University of, 3:12, 4:7, 

4:11, 4:18, 4:20, 5:12, 6:27, 

6:33,6:38,7:8,7:9,8:50, 11:41, 




Chicago, University of, 12:32, 
13:1. See also Metallurgical Lab- 
oratory, 29 

Clinton Engineer Works, 7:8, 
10:33, 10:42, 11:44, 11:45, 13:1 

Clinton, Tenn., 7:1, 8:34, 6:44 

Cloud chamber, 1:7, 12:35, A-l 

Columbia University, 1:53, 3:6, 
3:7, 3:10, 3:12, 4:2, 4:8, 4:9, 
4:13, 4:17, 4:22, 4:32, 4:38, 
4:39, 4:40, 4:41, 5:12, 8:56, 
10:1, 10:23, 10:26, 10:39, 11:41, 

Consolidated Mining and Smelting 
Co., 9:41 

Controls, 7:27, 8:8 

Cornell University, 3:12, 6:28 

Cooling, 6:43, 7:13, 7:15, 7:16, 
7:18, 7:26, 8:8, A-5 

Corrosion, 7:21, 7:25, 7:38, 10:17, 

Counter, Geiger-Muller, 1:7, 4:6, 
12:35, A-l 

Counter, proportional, A-l, A-3 

Cross Section, nuclear, 1 :45ff., 
4:6, 12:12, A-5. See also nuclei 

Cyclotron, 1:40, 4:10 

Danger coefficients, 8:28 

Decontamination, 8:23 

Detection of nuclear particles, 4:6, 
4:13, 8:39, 8:67, 12:35, A-l. 
See also Cloud Chamber, Coun- 
ter, Electroscope, lonization 

Deuterium, see Heavy water 

Deuteron, 1:21, 1:40, 6:34 

DSM Project, 5:23, 7:4 

du Pont, E. I., de Nemours and 
Co., Inc., 6:13, 6:18, 6:40, 7:4, 
7:5, 7:6, 8:50, 9:40, 10:29, 
11:19, 7:50ff. 

Electrolytic method of isotope 

separation, 9:25 

Electromagnetic methods of iso- 
tope separation, 4:30, 5:7, 5:15, 
9:28, ll:lff. Calutron, 11:2, 
11:10, 11:20. Ion source, 9:30, 
11:4, 11:5, 11:25. Isotron, 9:31, 
11:2, 11:24. Magnetron (ion 
centrifuge), 9:31, 11:2, 11:29. 
Space charge effects 9:30, 11:5, 

Electron, 1:11, 1:12, 1:23 
Electroscope, 1:7, 12:35, A-l 
Energy, conservation of, 1:2 
Enrichment factor, 9:8, 9:14, 9:20, 

9:30, 10:6, 10:7, 11:14, 11:21 
Equilibrium, radioactive, 8:18 

Fast fission effect, 8:9 

Feasibility Report, 6:6, 6:39, 6:43, 

Fission, 1:3, 1:53, 1:55, 1:57, 2:3, 
2:14, 2:19, 2:21, 2:35, 6:38, 
12:16. See also Neutron, Plu- 
tonium, Uranium 

Fission Products, 1:53, 1:56, 1:57, 
2:10, 2:32, 4:6, 7:29, 7:35, 8:17, 

Fractional distillation method of 
isotope separation, 9:16, 9:38 

Gamma rays, 1:9, 1:15, 1:19, 2:32, 
8:5, 8:17, A-l 

Gaseous diffusion method of iso- 
tope separation, 4:32, 4:37, 
4:47, 4:49, 5:6, 5:12, 5:14, 
9:14, 10:lff., 10:14, 10:26, 11:16, 

General Electric Co., 11:19, 11:45 

George Washington University, 

Graphite, 2:9, 2:19, 2:30, 2:31, 
2:36, 3:5, 3:7, 3:11, 4:2, 4:4, 



Graphite, 4:6, 4:13, 4:42, 4:47, 
5:14, 6:8, 6:28, 6:29 

Half Life, 1:16, 6:28, 8:17. See also 
Nuclei involved 

Hanford Engineer Works, 7:8, 
7:12, 8:50, 10:78, 13:1 

Harshaw Chemical Co., 6:13, 

Health hazards, 2:32, 2:36, 4:26, 
6:5, 7:16, 7:19, 7:26, 7:28, 7:35, 
7:36, 7:41, 8:35, 8:54, 8:63, 
8:67, 8:78, A-5. See also Radio- 
active poisons 

Heavy water, 3:14, 5:6, 5:12, 
5:14. As moderator, 2:10, 2:13, 
2:28, 4:38, 4:47, 5:14, 6:8, 
6:43, 6:45, 8:32, 8:55. Produc- 
tion, 4:39, 9:36 

Helium, see Alpha particle cooling 

Hooker Electrochemical Co., 10:29 

Houdaille-Hershey Corp., 10:26 

Indiana, University of, 4:11, 6:38 

Indium, 4:6 

Iodine, 4:6 

Ion accelerator, 1:40. See also 

lonization chamber, 1:7, 4:6, 

12:35, A-l 
Iowa State College, 3:12, 6:16, 

6:33, A-4 
Isobars, 1:14 
Isotopes, 1:14, 1:29. Separation 

of, 2:12, 5:7, 5:15, 6:2, 9:lff., 


J. A. Jones Construction Co., Inc., 
10:24, 10:41 

John and Mary Markel Founda- 
tion, 11:12 

Johns Hopkins University, 1:53, 

Joint New Weapons Committee, 

Kaiser Wilhelm Institute, 3:6 
Kellex Corp., 9:32, 10:2, 10:24, 
10:26, 10:41 

Lattice, 2:11, 2:19, 2:31, 4:4, 4:6, 
4:8, 4:13, 4:15, 4:16, 4:20, 4:22, 
4:23, 4:27, 5:15, 6:21, 6:22, 
6:28, 6:42, 6:43, 7:1, 7:3, 7:18, 
7:19, 7:46, 8:4, 8:9, 8:21, 8:28, 
8:36, 8:59, A-4. Poisoning of, 

Lithium, 1:19, 1:38, 1:48, 2:10, 

Los Alamos, N. M., 7:48, 12:4, 
12:18, 13:1, A-6 

Mallinckrodt Chemical Works, 

6:12, 6:18, 6:19, 6:22 
Manhattan District, 5:23, 5:29, 

5:30, 5:32, 6:18, 7:4, 10:23, 

11:11, 11:39, 12:2, 12:7, 13:1 
Mass, conservation of, 1:2, A-l 
Mass-Energy equivalence, 1 :4, 1 :6, 

1:8, 1:38 

Mass number, 1:13, 1:15, 1:35 
Mass spectrograph, 1:35, 9:28ff. 
Metal Hydrides Co., 4:41, 6:10, 

6:15, A-4 
Metallurgical Laboratory, 6:2, 

6:33, 6:35, 7:1, 7:9, 7:10, 7:40, 

7:46, 7:50, 8:2, 8:22, 8:34, 

8:56, 11:2, 11:19, 12:2, 12:32. 

See also Chicago, University of 
Military Policy Committee, 5:25, 

Minnesota, University of, 3:12, 

Moderator, 2:8ff., 2:11, 2:13, 

2:14, 2:19, 2:20, 2:28ff., 4:1, 



Moderator, 4:8, 8:8, 12:40. See 

also Beryllium, Graphite, Heavy 

Multiplication factor, 4:13, 4:15, 

4:16, 6:10, 6:11, 6:22, 7:19, 

8:12, 8:15, A-4 
M. W. Kellogg Co., 10:2, 10:23, 

10:32, 10:11. See also Kellex 


National Bureau of Standards, 
6:16, 6:20, 6:38, 9:31 

National Carbon Co., Inc., 6:20, 
10:23, A-4 

NDRC, 2:37, 3:4, 3:9, 3:10. 
Section S-l, 3:14, 4:14, 4:31, 
5:2. See also Advisory Commit- 
tee on Uranium 

Naval Research Laboratory, 4:36, 
11:38, 11:39, 11:47 

Navy Department, 2:1, 3:4, 3:6, 
3:11, 3:12, 4:36, 10:2, 11:39, 

Neptunium, 1:58, 2:19, 6:34, 8:18 

Neutron, l:18ff., l:23ff., 1:33, 
Absorption of, 1:47, 2:13, 1:57, 
2:3, 2:8ff., 2:12, 2:19, 8:4, 8:6, 
12:8, 4:8, 4:19, 6:20ff., A-5. 
Delayed, 6:23, A-3, A-4. Detec- 
tion, see Detection of nuclear 
particles. Fast, 1:57, 2:1, 2:21, 
2:14, 2:10, 2:12, 4:25, 6:37, 8:9. 
Fission induced by, 1:52, 1:57, 
2:21, 3:6, 4:6, 4:24, 4:25, 10:1. 
Resonance, 2:12, 4:6, 4:8. 
Sources, 1:40, 4:10, 4:14, 8:31, 
8:32, 12:38, A-5. Thermal, 
1:57, 2:1, 2:9, 2:21, 2:14, 3:6, 
4:6, 4:25, 8:31, 8:39, 10:1, 
12:40, A-l 

Nitrogen, 1:17,1:51 

Nuclear disintegrations, 1:15, 1:20, 
1:38, 1:49. Artificial, 1:17 

Nuclear Reaction, l:38ff. Nota- 
tion for 1:43. See also Fission, 
Nuclear disintegration, Nuclei 

Nucleus, 1:11, 1:12. Structure of, 

Oak Ridge, Tenn., see Clinton 
Engineer Works 

OSRD, 5:2, 5:9, 10:2, 10:23, 
11:26, 12:2. Section S-l, 5:3, 
5:17, 5:28, 7:1, 10:33, 11:4, 
11:18. See also Advisory Com- 
mittee on Uranium 

Oxygen, 1:17, 1:51 

Philadelphia Navy Yard, 11:39, 

Pile, 2:20. See also Argonne Labo- 
ratory, Clinton Engineer Works, 
Hanford Engineer Works, Lat- 
tice, West Stands pile 

Planning Board, 5:5, 5:6, 6:8 

"Pluto," 8:68 

Plutonium, 1:58, 2:23, 2:36, 3:17, 
4:24, 4:25, 5:7, 5:21, 6:1, 6:4, 
6:34, 6:35, 8:lff., 12:10. Fission, 
1:58, 2:14, 4:24, 4:48. Produc- 
tion, 2:19, 5:14, 6:32, 6:41, 7:3, 
7:8ff., 7:18ff., 8:lff., 9:44, 11:32, 
2:18. Radioactivity, 1:58, 4:24 

Poison gas, 4:27 

Polonium, 1:19 

Positron, 1:21, 1:51 

Power, 1:48, 2:3, 2:16, 2:34, 3:5, 
3:14, 4:48, 6:29, 7:26, 13:5 

Princeton University, 3:12, 4:7, 
4:9, 4:11, 4:12, 4:18, 4:22, 5:12, 
9:40, 11:2, 11:44 

Protoactinium, 1:57, 2:21 



Proton, 1:13, 1:17, 1:19, 1:23, 
1:24, 1:33, 1:38, 1:40, 1:48, 
1:51, 4:10, A-l 

Purdue University, 6:38 

Radioactivity, 1:6, 1:7, 1:15, 2:32. 
Artificial, 1:26, 1:30, 4:6, 4:26, 
A-l. Elements showing, 1:16. 
Measurement of, see Cloud 
chamber, counter, Detection of 
nuclear particle, Electroscope, 
lonization chamber. Poisonous 
effects, 4:26, 4:48; see also 
Health. Products of, see Alpha 
particles, Beta particles, Gam- 
ma rays, Nuclear disintegra- 
tion, Radioactive series 

Radium, 1:15, 1:16, 2:24, 2:32 

Reference Committee (National 
Research Council), 3:3 

Reflector (tamper), 12:13, 12:24, 

Relativity, 1:4 

Relaxation time, A-4 

Reproduction factor, see Multipli- 
cation factor 

Research Corporation, 1 1 :4 

Resonance escape probability, 8:10 

Reviewing Committee (National 
Academy of Sciences) 3:15, 
3:16, 3:18, 3:21, 3:23, 4:23, 
4:24, 4:27, 4:48, 6:6, 6:39, 7:1 

Rhodium, 4:6 

Rice Institute, 6:38 

Richland, Wash., see Hanford En- 
gineer Works 

Rockefeller Foundation, 11:12 

Scintillations, A-l 
Shielding, 7:28, 8:67 
"Sneezy," 8:68 
Speer Carbon Co., 6:20, A-4 

Standard Oil Development Co., 

3:12, 5:12, 9:41, 9:42 
Stanford University, 6:38 
Stone and Webster Engineering 

Corp., 10:34, 11:19, 11:45 

Tamper, see Reflector 

Tennessee Eastman Corp., 11:19, 

11:34, 11:45 

Thermal diffusion method of iso- 
tope separation, 4:36, 9:21, 

Thermal utilization factor, 8:11 
Thorium, 1:15, 1:16, 1:57, 2:21, 

2:24, 2:25 

Tolman Committee, 13:5, 13:6 
Top Policy Group, 3:22, 5:9 
Transuranic elements, 4:25 

Union Miniere, 3:7 

Union Carbide and Carbon Corp., 

United States Graphite Co., 4:42 

Units, A-l 

Uranium, 1:7, 1:15, 2:23, 2:24, 
2:26, 2:27, 6:4, 6:34, 11:36. 
Fission, 1:3, 1:52, 1:57, 2:1, 2:3, 
2:10, 2:12, 2:13, 2:14, 2:32, 3:6, 
8:8ff., 2:35, 2:36, 4:49, 8:5, 
10:1, 8:9, 3:16. Isotopes, 1:16, 
2:18, 2:4, 4:34, 4:47, 5:21, 6:1, 
10:1, 4:24. Isotope separation, 
see Isotope separation. Neutron 
absorption, 2:3, 2:6, 2:11, 2:19, 
4:2, 4:6, 4:7, 4:24, 4:25, 8:5. 
Uranium production, 4:41, 6:10, 

Uranium Bromide, 1 1 :4 

Uranium Committee (NDRC), 
3:9, 3:14, 4:36, 9:30, 11:26, 
11:29. See also Advisory Com- 
mittee on Uranium 

264 Index 

Uranium Hexafluoride, 4:33, 4:36, Washington University (St. Louis), 
9:14, 10:3, 10:11 6:34 

Westinghouse Electric and Manu- 
Virginia, University of, 3:12, 5:12, facturing Co . } 6:10 , 6:13, 6:14, 

9:42 9:42, 11:19, 11:45, A-4 

War Department, 7:5, 3:6, 3:12, West Stands pile, 8:27, A-4 
5:9, 11:18 Wisconsin, University of, 6:38